The chemistry of Photography requires the attention, in a greater or less degree, of every practitioner. It is of the utmost importance, that those who wish to meet with success in the various processes given, should not only be provided with a good selection of chemicals, but also understand the nature of the agent employed. To give a perfectly complete and full list of every agent used would require more time and space than can be given in this work. I shall confine myself to some of the most important, and to such articles as are of the greatest interest to the practitioner.

Soluble Cotton.

I have, in my practice and trade, adopted the term soluble cotton as the one most appropriate, making a desirable distinction from the article sold as gun cotton, they being of a somewhat different nature—gun cotton being the most explosive and least soluble, while the other preparation is more soluble and less explosive.

There are two methods employed in the preparation of soluble cotton; one being by the use of nitric and sulphuric acids, and the other with sulphuric acid and nitrate of potash. The last of these I would recommend as being the most convenient for those who require only a small quantity of cotton. Persons experimenting in the preparation of this article should exercise much care and judgment. A good cotton is not the result of hap-hazard operation. The operator should be acquainted, as nearly as possible, with the quality of the chemicals employed, and the proper mode of manipulation.

Articles necessary.—One quart Wedgewood mortar and pestle, or evaporating dish; one glass rod; one pane of glass, large enough to cover the mortar or dish; one ordinary-sized pail two-thirds full of pure rain or distilled water, and at least ten times that quantity of water at hand; twelve ounces (by weight, avoirdupois) of nitrate of potash (Dupont's refined, pulverized); twelve ounces (by measure) of commercial sulphuric acid; and three hundred and forty grains of clean, pure cotton wool.

Remarks.—It is advisable that the mortar or dish be deep and narrow, as the mixture can be better formed in a vessel of this shape. If not convenient to procure a mortar, a common earthen bowl will answer; glass is objectionable, as the heat generated in the combination of the acid and nitre is liable to crack it. A new pail should not be used, especially if it is painted, as the acids attack the paint, and injure the cotton. I prefer one that has been used for some time, and has been frequently cleaned. A common earthen wash-bowl, or any large glass dish, will answer in place of the pail. Metal pails or vessels should not be used.

Nitrate of Potash (saltpetre) should be dry and finely-powdered. I use none other than Dupont's refined; this is very nearly, if not absolutely, chemically pure.

The commercial Sulphuric Acid (oil of vitriol) of America is of great uniformity of strength, as sold by druggists generally. I use a test-bulb graduated to the proper density, and have been very successful in my experiments.

In some twenty different samples of acid, used in different cities in the United States, I found only one that produced a poor cotton, and this might have been influenced by the moisture of the atmosphere, it being a very rainy day when I used it.

During my recent and somewhat extensive practice, I have thought that the fine long fibres of cotton wool do not make so desirable a soluble cotton as that which is heavy or common. Four or five very careful experiments upon this point, have had the effect to produce in me a strong belief that my ideas are entitled to some consideration. I should not select the finest cotton for making soluble cotton, but now invariably take that which is thick or coarse.

The result of my experience is (other things being equal), that cotton prepared in fine dry weather has a greater degree of solubility than when prepared in a moist atmosphere: hence I would recommend the experimenter to choose fine, clear weather for preparing it.

Manipulation.

Having at hand every article requisite, proceed as follows:—Put the nitrate of potash into the mortar or dish; be sure it is dry and well powdered, and then add the acid; stir them well with the pestle and glass rod, so that the lumps will be all (or nearly so) out, and a pasty solution formed. This operation should not occupy more than two minutes' time. Then put in the cotton, about one-quarter of the whole bulk at a time: it should be well picked apart, so that it may come immediately in contact with the acids, and should be kneaded, with the pestle and glass rod, into the solution, and as soon as wetted, another quarter should be added and wetted as soon as possible; so continue until all is in: then knead with the pestle and mortar for at least four minutes, or until every fibre of the cotton is saturated with the liquid; then the mortar should be covered over with the pane of glass, and allowed to stand for fifteen or twenty minutes; then the entire contents of the mortar should be thrown into the pail two-thirds full of water, and stirred with the glass rod as rapidly as possible: if this rapid stirring is omitted, the cotton will be injured by the action of the acids in combining with the water. The water should be poured off, and another change put into the pail.

After about three changes, the hands may be used in the farther washing. The hands should be perfectly clean, and free from all chemicals. The changes of water and washing should be continued until every trace of acid has disappeared, which can be seen by testing with blue litmus test-paper. After it is thought that the cotton has become free, the water may be squeezed out of a little lump about the size of a pea, and then placed between the fold of the test-paper, and if it reddens the paper, there is acid present, and the washing should be continued until there is no change in the paper. When this is done, the cotton can be put into the folds of a dry towel or cloth (which has been thoroughly rinsed, so that no soap be present), and wrung out as dry as possible, and then it may be picked apart and put aside, exposed to a moderate temperature (say 100° Fahr.) to dry, when it is ready for use.

I employ the method (for convenience, nothing more) of displacing the water by the use of alcohol. [Cutting's patent—see patents.] I wring out the water as before, then place the cotton in strong alcohol, stir and press it, and then pour it off; wring it out again, then put it in a change of alcohol, let it soak for about five minutes, then wring it out as dry as possible, pick it apart, and it will dry immediately, and place it in a close stoppered bottle; or, if wanted for use at once, put it into the dissolving solution immediately.

I will here remark that, since the first edition, I have had occasion to use large quantities of soluble cotton, and have found that if it be kept in an atmosphere of alcohol and ether, its solubility is somewhat improved: that is, in the case of its not being used immediately after its preparation. This is easily kept, by dropping a few drops of ether or alcohol into the bottle containing it, and then sealing close until wanted for use. In the event of the water being displaced by alcohol, it is not necessary to thoroughly dry it, but put in a perfectly close bottle to keep.

Remarks.—There are a few precautions necessary to be observed in the preparation of soluble cotton. I should select a fine clear day, if time is no object; nevertheless I have made a good article in a moderately dense atmosphere. Sulphuric acid has a powerful affinity for hydrogen, consequently, in damp weather, it is more or less reduced by the moisture in the air.

It is advisable to prepare the nitro-sulphuric acid mixture on a roof, or between two doors or windows, where there is a good current of air, in order to prevent the inhalation of white vapors which arise, and are very poisonous to the lungs. As a preventive, in case of inhaling these vapors, I apply the fumes of aqua-ammonia. It is best for every one to have six or eight ounces of this always at hand; it neutralizes all acid that maybe spattered on the clothes, prevents its destructive powers, and restores the color.

Yellow vapors sometimes appear when putting the cotton in contact with the solution: this arises from its not being wet; and when they do appear, the cotton where they are should be quickly put under the liquid and kneaded rapidly, which will prevent a continuance of these vapors. I have had them appear, and used the cotton, and could not observe that any bad effect had been produced.

The temperature is worthy the attention of the operator: if it be low, as in winter, and the cotton be left in the nitro-sulphuric mixture for fifteen or twenty minutes, the whole becomes a thick, stiff mass, bedded together, and has not had proper action, giving a bad article. A good temperature is about 140° Fahr. for the last of the time the cotton is in the mixture. This is not always convenient; so the operator will be governed by circumstances, taking his chance of having a good article. In some cases I have heated a thick iron plate, at a moderate temperature, placing the mortar upon it, and thus aided in regulating the temperature. This is the most convenient method I have employed.

It has been thought advisable to publish in full the account of Edw. Ash Hadow's experiments and investigations upon the subject of soluble cotton. The following is an account of them as it appeared in Humphrey's Journal, vol. VI. p. 12:—

"Having, in my earlier experiments on the collodion process of photography, experienced some difficulty in always producing a collodion of uniform quality with regard to sensitiveness, tenacity and fluidity, although making use of the same materials for its preparation, and this I find being the complaint of many others, it has been my study lately to determine the variations in quality to which the ingredients are liable, and the effects of these variations on the sensitive film, and likewise to ascertain whether the excellent qualities of some samples of collodion depend on the materials in ordinary use, or on some substances accidentally or intentionally added. Researches in the preparation of collodion may appear superfluous, now that it is supplied of the best quality by so many makers; but as some persons of an independent turn of mind still prefer manufacturing their own, I venture to bring forward the subject with the hope of benefiting them. In this beautiful process so much depends for success on the quality of the collodion, that when in possession of a good specimen, it becomes one of the easiest and most simple, and ought to be the most certain of all the processes yet devised; for here no material of uncertain composition is introduced, such as paper, and thus we have nothing to fear from plaster of Paris, alumina, or specks of iron or copper, which continually endanger or modify the calotype process; each ingredient can and ought to be obtained in a state of perfect purity, and with this precaution the degree of success depends upon the skill of the operator himself.

"Of all the substances used in this process, the gun-cotton is usually the only one actually prepared by the operator himself; in this he cannot fail to have observed the great variations in the solubility, and, when dissolved, the transparency and tenacity of the films, to which it is liable; the various processes also that are given appear at first sight unaccountably different, some directing ten minutes, others a few seconds immersion. In consequence of this I have specially examined into the cause of all these variations, with a view to obtain certainty, and also have endeavored to discover how far they affect the sensitiveness of the prepared surface. If we take a mixture of the strongest nitric and sulphuric acids and immerse as much cotton as can be wetted, after some minutes squeeze out the acid as far as possible, then immerse a second portion of cotton, and again express the acids for a third portion of cotton, and so on until the liquid is exhausted, we shall find, on comparing the cottons thus treated, after washing and drying, that there is a gradual alteration in their properties, the first being highly and perfectly explosive, and each succeeding portion less so, until the portion last immersed will be found hardly explosive, leaving distinct traces of charcoal or soot when burned. This may not appear surprising at first sight, as it may be imagined that the latter portions are only a mixture of gun-cotton and common cotton; this is, however, not the case, for if each quantity be immersed sufficiently long, it will not contain a fibre of common cotton, and may yet become charred on burning like unaltered cotton. The most remarkable difference, however, is discovered on treating them with ether containing a little alcohol, when, contrary to what might have been anticipated, the first or strongest gun-cotton remains untouched, while the latter portions dissolve with the utmost ease, without leaving a trace behind, which alone is sufficient proof that no unaltered cotton remains. This difference in properties is owing to the gradual weakening of the acid mixture, in consequence of the nitric acid being removed by the cotton, with which it becomes intimately combined, at the same time that the latter gives out a proportionate quantity of water. In consequence of these experiments, a great many mixtures of these acids were prepared of various strengths, each being accurately known, both to determine whether there were more than one kind of soluble gun-cotton, and, if there were, to ascertain exactly the mixture required to produce the most suitable to photographic purposes. By this means, and by, what I believe has not been pointed out, varying the temperature, at least five varieties were obtained;—first, gun-cotton, properly so called, as before stated, quite insoluble in any mixture of alcohol and sulphuric ether. Secondly, an explosive cotton, likewise insoluble, but differing chemically from the first, obtained by a mixture of certain strength when used cold. If warm, however, either from the heat produced spontaneously on mixing the two acids; or by raising the temperature artificially to about 130°, the cotton then immersed becomes perfectly soluble, producing a third variety; if, however, it be thoroughly dried, it becomes in a great measure insoluble. The fourth is obtained by the use of weaker acids used cold, and the fifth when the mixture has been warmed to 130° previous to the immersion of the cotton; in either of the two last cases the product is perfectly soluble, but there is a remarkable difference between their properties, for on dissolving 6 grains of each in 1 ounce of ether, the cotton treated with warm acids gives a perfectly fluid solution (which is likewise the case with the third variety produced by acids something stronger), while that obtained by the use of cold acids makes a mixture as thick as castor-oil.

"Having obtained these more strongly marked varieties, as well as intermediate kinds, with all gradations of solubility, it was necessary, before I could select any particular formula for preparing the cotton, to compare their photographic properties, with especial reference to sensitiveness, opacity of the reduced silver in negatives, and its color in positives. A certain weight of each being dissolved in a portion of the same mixture of alcohol and ether previously iodized, the comparison was made, by taking the same objects with each collodion in succession, and likewise by pouring two samples on the same plate of glass, and thus exposing them in the camera together, side by side; this last proved to be much the most satisfactory plan, and was repeated many times for each sample, taking care to reverse the order in which they were poured on, that there might be no mistake arising from the difference of time elapsing between the pouring on of the collodion and its immersion in the sensitive bath. By these experiments I had confidentially hoped to have solved the question as to the cause of difference in sensitiveness and other photographic properties of collodion; but in this I was disappointed, for, after repeated experiments, I believe I may safely affirm that they are precisely similar as regards their photographic properties. The same I believe may be said of Swedish paper collodion, judging from a few comparative experiments I have made, and indeed it is difficult to discover what is the superiority of this material over clean cotton-wool; the ease of manipulation which some allege is a matter of taste, but I should decidedly prefer the open texture of cotton to that of a substance like filtering paper, composed of a mass of compacted fibres, the innermost of which are only reached when the acids have undergone a certain degree of weakening by the water abstracted from the outer fibres; and when we consider that from cotton alone we have the means of preparing all varieties of collodion, from the most powerfully contracting and transparent to the weakest and most opaque, and each if required with equal and perfect certainty, there appears to be choice enough without resorting to another material, differing only in being more rare and more difficult to procure. But, although the photographic properties of these varieties of collodion-wool are so similar, other circumstances, such as fluidity, tenacity, and transparency, render its preparation of some importance, and indicate that the acid mixture should always be used warm; and it is chiefly in consequence of this very circumstance, that greater success attends the use of nitrate of potash and sulphuric acid than that of mixed acids; for the former when mixed, produce the required temperature, and must be used while warm, since on cooling the mixture becomes solid, whereas acids when mixed do not usually produce so high a temperature, and being fluid can be used at any subsequent period; another obstacle to their use is the great uncertainty of the strength of the nitric acid found in the shops, requiring a variation in the amount of sulphuric acid to be added, which would have to be determined by calculation or many troublesome trials. When a proper mixture is obtained, the time of immersion is of no importance, provided it be not too short, and the temperature be maintained at about 120° or 130°; ten minutes is generally sufficient; (though ten hours would not render the cotton less soluble, as is sometimes asserted.)

"In using the mixed acids, the limits are the nitric acid being too strong, in which case the product is insoluble, or too weak, when the cotton becomes immediately matted or even dissolved, if the mixture is warm. I have availed myself of these facts in order to produce collodion-wool by the use of acids, without the trouble of calculating the proper mixture according to their strength. Five parts by measure of sulphuric acid, and four of nitric acid of specific gravity not lower than 1·4, are mixed in an earthenware or thin glass vessel capable of standing heat; small portions of water are added gradually (by half drachms at a time, supposing two ounces to have been mixed,) testing after each addition by immersion of a small portion of cotton; the addition of water is continued until a fresh piece of cotton is found to contract and dissolve on immersing; when this takes place, add half the quantity of sulphuric acid previously used, and (if the temperature does not exceed 130°, in which case it must be allowed to cool to that point,) immerse as much cotton, well pulled out, as can be easily and perfectly soaked; it is to be left in for ten minutes, taking care that the mixture does not become cold, and then transferred to cold water, and thoroughly washed; this is a matter of much importance, and should be performed at first by changing the water many times, until it ceases to taste acid, and then treating it with boiling rain-water until the color of blue litmus remains unchanged; the freedom from all trace of acid is insured by adding a little ammonia before the last washing. Cotton thus prepared should dissolve perfectly and instantaneously in ether containing a little alcohol, without leaving a fibre behind, and the film it produces be of the greatest strength and transparency, being what M. Gaudin terms 'rich in gun-cotton.'

"The mixture of nitrate of potash and sulphuric acid is defective chiefly from the want of fluidity, in consequence of which the cotton is less perfectly acted on; this may be remedied by increasing the amount of sulphuric acid, at the same time adding a little water; a mixture of 5 parts of dried nitre, with 10 of sulphuric acid, by weight, together with 1 of water, produces a much better collodion wool than the ordinary mixture of 1 of nitre with 1½ of sulphuric acid. The nitre is dried before weighing, in order that its amount, as well as that of the water contained in the mixture, may be definite in quantity; it is then finely powdered, mixed with the water, and the sulphuric acid added; the cotton is immersed while the mixture is hot, and afterwards washed with greater care even than is required when pure acids are used, on account of the difficulty of getting rid of all the bisulphate of potash that adheres to the fibres, which both acts as an acid and likewise causes the collodion to appear opalescent when held up to the light; whereas the solution should be perfectly transparent."

Plain Collodion.

To dissolve the soluble cotton (pyroxyline), and form plain collodion, proceed as follows:

Take of

Soluble cotton enough to give the solution a consistency such as will allow it to flow evenly over the surface of the glass, and impart to it quite a thick and transparent coating. If the coating is opaque, the cotton has not been properly prepared, the acid mixture has been too weak.

Remarks.—It is desirable for every operator to use chemicals of uniform strength, and the better method to adopt is to employ those purchased from some one respectable manufactory, and not take those furnished by irresponsible and unconscientious parties. At least one-half of the failures experienced by beginners is from want of good chemicals. It is not economy to purchase a cheap article.

Alcohol is an article that can be procured in almost any small village in the United States, and is in general fit for collodion purposes. I have used 88 per cent, in the above proportions, also the intermediate varieties to 98 per cent., and have been quite successful; but feel convinced that the ordinary 98, as marked (which usually stands by actual test 95 to 97 per cent.), is preferable, except in cases where water is employed in dissolving the iodizing salts, when I would use fully 98 per cent.

Before concluding the subject on plain collodion, I will introduce the account given by Mr. E. A. Hadow of his interesting and valuable experiments, as published in Humphrey's Journal, Vol. VI, page 18.

"Having obtained good collodion-wool, the next point of inquiry was with regard to the solvent; to ascertain whether the addition of alcohol beyond what is necessary to cause the solution of the gun-cotton in ether, were beneficial or otherwise. For this purpose ether and alcohol were prepared perfectly pure, and mixtures were made of 1 of alcohol to 7 of ether, 2 to 6, 3 to 5, 4 to 4 and 5 to 3. In one ounce of each were dissolved 6 grains of gun-cotton and 4 grains of iodide of ammonium (iodide of potassium could not be employed, since it requires a certain amount both of water and alcohol to keep it in solution); they were then compared, using a 35-grain solution of nitrate of silver, both by pouring on separate glasses, and likewise by covering two halves of a plate with two samples, as in examining the gun cottons, thus placing them under the same circumstances during the same time; in this way the effect of adding alcohol was very clearly perceived, since the difference between the collodions was much greater than could have been anticipated.

"The first mixture containing only ¹/₈th of alcohol was quite unfit for photographic purposes, from its being almost impossible, even with the most rapid immersion, to obtain a film of uniform sensitiveness and opacity throughout, the surface generally exhibiting nearly transparent bands, having an iridescent appearance by reflected light.

"The second mixture with ¹/₄th of the alcohol is liable to great uncertainty, for if there be any delay in pouring off the collodion the same appearances are seen as in the first, and like it the surface is very insensitive to light, while if the plate be rapidly plunged in the bath, the collodion film becomes much more opaque than before, and is then very sensitive.

"The third proportion of 3 of alcohol to 5 of ether is decidedly the best, giving without the least difficulty a beautifully uniform and highly sensitive film, at the same time perfectly tough and easily removable from the glass if required. A further addition of alcohol, as in the two last collodions, was not attended with any corresponding advantage or increase of sensitiveness; on the contrary, the large proportion of alcohol rendered them less fluid, though with a smaller quantity of gun cotton they would produce very good collodions, capable of giving fine films: the cause of the weakness of the film, observed on adding much of the ordinary alcohol, is the large amount of water it usually contains.

"This surprising improvement, caused by the addition of a certain quantity of alcohol, is referable to causes partly chemical, partly mechanical, for, on examining the films, it will be found in the first, and occasionally in the second collodion, that the iodide of silver is formed on the surface, and can be removed entirely by friction without destroying the transparent collodion film below, while in those collodions that contain more than one-fourth of alcohol, the iodide of silver is wholly in the substance, and in this state possesses the utmost sensitiveness.

"This difference of condition is owing to the very sparing solubility of ether in water, which in the first case prevents the entrance of the nitrate of silver into the film, consequently the iodide and silver solutions meet on the surface; but on addition of alcohol, its solubility enables the two to interchange places, and thus the iodide of silver is precipitated throughout the substance in a state of the utmost division. This difference is clearly seen under the microscope, the precipitate being clotted in the one case, while in the other the particles are hardly discoverable from their fineness.

"The presence of a little water considerably modifies these results, since it in some degree supplies the place of alcohol, and is so far useful; but in other respects it is injurious, for, accumulating in quantity, if the collodion is often used, it makes the film weak and gelatinous, and what is worse, full of minute cracks on drying, which is never the case when pure ether and alcohol are used. Since the ether of the shops almost always contains alcohol, and frequently water, it is important to ascertain their amount before employing it for the preparation of collodion; the quantity of alcohol may be easily ascertained by agitating the ether in a graduated measure glass (a minim glass does very well) with half its bulk of a saturated solution of chloride of calcium; this should be poured in first, its height noted, and the ether then poured on its surface, the thumb then placed on the top, and the two agitated together; when separated, the increase of bulk acquired by the chloride of calcium indicates the quantity of alcohol present, and for this allowance should be made, in the addition of alcohol afterwards to the collodion.

"Water is readily detected, either in ether or alcohol by allowing a drop to fall into spirits of turpentine, with which they ought to mix without turbidity; this is immediately produced if they contain water: for detecting water in alcohol, benzole is a more delicate re-agent than spirits of turpentine (Chemist, xxix, 203). It is also necessary that ether should be free from a remarkable property it acquires by long keeping, of decomposing iodides and setting free iodine, which thus gives the collodion a brown color; the same property may be developed in any ether, as Schonbein discovered by introducing a red hot wire into the vapor in the upper portion of a bottle containing a little ether and water; if it be then shaken up and a solution of iodide poured in, the whole rapidly becomes brown; this reaction is very remarkable and difficult to explain for even a mixture of the ether and nitric acid fails to produce a color immediately. Ether thus affected can only be deprived of this property by rectification with caustic potash."

Bromo-Iodized Collodion for Positives.—No. 1.

One very important object in connection with this part of the collodion process is to have chemicals of a good quality, and always employ those of a fixed standard.

Double iodide of potassium and silver (see page 62) enough so that when the plate comes from the nitrate of silver bath, it will have an opaque cream color.

Remarks.—In the preparation of this sensitive collodion, it is necessary to be cautious and not add too much of the iodide of potassium and silver, for in that case the coating would flake off, and falling into the silvering solution, the operator would be obliged to filter it before he could silver his plate with safety as regards spotting it.

The method I employ is to add the plain collodion, bromide and iodide of potassium and silver, iodide of ammonium and hydro-bromic acid, and then cautiously add the double iodide of potassium and silver from five to ten drops at a time, trying the collodion from time to time by pouring a little on a narrow strip of glass, which I dip into the silvering solution, and let it remain for two minutes. If the coating assumes the proper color (a cream color), I shake the contents of the bottle, and then stand it aside to settle: it is better after it has stood for a week or two.

This collodion I have used after it has been made eight months, and produced fine and satisfactory results, and use this nearly altogether in practice. Since the first edition of this work has been issued, I have sold over two thousand pounds of this preparation, and the demand is on the increase. I will append another preparation (No. 2) which I have successfully employed, and some operators prefer.

Bromo-Iodized Collodion for Positives.—No. 2.

Enough of the double iodide of potassium and silver to give the coating a cream color when it comes from the silvering solution. It will take from one to three drachms. Or this last may be omitted, and a few drops of a saturated solution of dry iodine in alcohol may be added. Either of these plans have been successful in my practice.

Remarks.—The iodide of potassium being insoluble in the collodion, it should be first dissolved in as little water as possible; i. e., take the quantity, 30 grains, put it into a one-ounce graduate, and with a glass rod stir it, adding water, drop by drop, only until all of the salt is dissolved. Then it may be poured into the collodion, and there will be a white powdery precipitate.

The bromide of ammonium will dissolve in the collodion, and can be put into it. When all of the accelerators are in, it should be well shaken, and then allowed to settle and become clear. When wanted, a sufficient quantity may be poured into a vial (see Fig. 34) for use, and the main or stock bottle should not be disturbed oftener than necessary. This last collodion is not as durable as the first, but is less trouble to prepare.

Bromo-Iodized Collodion for Negatives.

This collodion should be allowed to stand and settle twenty-four hours before it is used: when wanted, it should be poured off into a collodion vial. The more free the collodion is from sediment and small particles of dust or undissolved cotton, the softer and more perfect will be the impression it makes.

In case the above proportions of iodide of potassium should not produce a cream-colored coating, when it comes from the nitrate of silver bath, more may be added: for example, if the coating is of a bluish tint, I would dissolve 6 grains of iodide of potassium in water, as before, and then try it: shake well, and test it by putting a little on a slip of glass, and dipping it into the silvering solution; if it coats to a cream-color, it is right.

It should be borne in mind, that after the addition of iodide of potassium here recommended, the collodion should be allowed to stand until settled, before undertaking to produce a picture, although the coating may be previously tested by means of a slip of glass.

Solution of Bromide and Iodide of Potassium and Silver.

Dissolve 130 grains of crystallized nitrate, of silver in 4 ounces of pure water, in a long 8-ounce vial. Then in a clean 1-ounce graduate, or some other convenient vessel containing half an ounce of water, dissolve 130 grains bromide of potassium. When this and the nitrate of silver are both dissolved, pour the solution of bromide of potassium into the vial containing the silver, and a thick yellow precipitate will fall. This is the bromide of potassium and silver. This should be washed by nearly filling the vial with water; shake it, and then let it settle, which it will readily do, and then pour on the water, leaving the yellow mass in the bottom of the vial; continue this operation of washing for at least ten changes of water; then, after draining off the water as close as possible, put into the vial four ounces of alcohol, shake it well and let it settle; then pour off as close as possible. By this means the water is nearly all taken out.

Pour into the vial three ounces of alcohol; then in a small mortar finely pulverize one ounce of iodide of potassium, and the solution, which was before clear, will be more or less of a yellow color, and the bulk of the yellow precipitate will be diminished. I have sometimes completely re-dissolved the yellow precipitate, but this does not often occur, except there be more water present than is advisable. It is better to have an excess of bromide of potassium in the solution. This can be seen by its being white, and remaining undissolved in the bottom of the vial. This solution should be prepared in the evening, or in a dark room, and only the light of a lamp or candle employed.

Double Iodide of Potassium and Silver.

This solution is made in the same manner as in the foregoing article, substituting the iodide of potassium for the bromide—no bromide being used in this preparation. The yellow precipitate in this case will be re-dissolved and taken up in the solution: it may require more than one ounce of pulverized iodide of potassium to effect this, but it may be added in excess, so that the solution shall contain a quantity in powder.

Developing Solution.

Put these into a quart bottle, and shake until the crystals are all dissolved, and this can be kept for a stock bottle, and when wanted for use pour into another bottle.

Shake this mixture well, and filter through a sponge, and it is ready for use. I file a mark in this bottle indicating five ounces, and another for 1 ounce: this will save time in mixing the solution.

Remarks.—In my recent tour of the United States, I found it difficult to obtain a good article of protosulphate of iron, and in its stead I used the common copperas, such as I could find almost in any store. I employ from one-fourth to one-half more than the quantity given above. If it looked a clear green, and free from a white or brownish powder, about one-fourth addition: i. e., four ounces, instead of three, as given above. If the solution in the stock bottle is not wanted for a week or more, a few crystals of the protosulphate of iron should be added, as it decomposes, and the strength is depreciated.

There is quite a difference in the strength of the acetic acid as sold by out country druggists, and the operator should be sure that he has No. 8, to which quality the above proportions are adapted. I never have employed the developing solution but once, but can see no objections to use it for a number of glass plates, but it should be filtered every time before using. The quantity of nitric acid may be increased, so long as a proper proportion is preserved with the strength of the bath. The effect of this addition of acid will be to brighten the impression; but if carried too far, the reduction (developing) will be irregular, and the harmony of the impression injured.

Fixing Solution.

Remarks.—I put enough of the cyanide of potassium into the water to make the solution of such strength as to dissolve off the iodide of silver ("coating") in from twenty to sixty seconds. The operation is quite similar to that of hyposulphate of soda upon the coating of the Daguerreotype plate. A too concentrated solution is likely to injure the sharpness of the image.

Brightening and Finishing the Image.

The article I now employ for finishing off my Positives is in market, and known as Humphrey's Collodion Gilding. It is a new preparation, and exerts a powerful influence upon the image, having the same brightening effect as chloride of gold on the daguerreotype. There is no article now in market that equals this. I have until quite recently used a varnish for this purpose, but having something that is of far greater value, I have discarded it. It is one of the most valuable improvements since the application of the Collodion Film as a vehicle for producing photographic images. It is a new discovery, and is being rapidly brought into use by the first ambrotypers and photographers in America. It adds at least one-half to the beauty of an ambrotype, above any method heretofore in use. It is imperishable, giving a surface almost equal in hardness to the glass itself. It is easy of application; it gives a brilliant finish; it is not affected by a moist atmosphere; it is not affected by pure water; it is the best article ever used for finishing ambrotypes; it will preserve glass negatives for all time; it will preserve the whites in the ambrotype; it gives a rich lustre to drapery; it will bear exposure to the hot sun; it preserves positives and negatives from injury by light. It is an article that, when once tried, the operator upon glass (positive, negative, or albumenized plates) will not do without.

The ingredients in the composition of this gilding are neither patented nor published, but it can be procured from any dealer in photographic chemicals.

Nitrate of Silver Bath.

I here give what I consider an improvement on the bath mentioned in the first edition of this work. I first published it in Humphrey's Journal, No. 23, Vol. VII.:

The nitrate of silver solution is an important mixture in the chemical department of the ambrotype process, and requires the especial care of the operator in its preparation. I give the following as one of the most approved for general practice. It is well adapted to the production of positives, and its action is of great uniformity.

This proportion is to be observed for any quantity of solution. If I were to prepare a bath 40 ounces, I would proceed as follows:

Measure the water, and put into a two-quart bottle; then pour out 8 oz. of it in a pint bottle, and into this put the whole of the nitrate of silver (1800 gr.); shake it well until it is all dissolved. This forms a concentrated solution—into which put the following prepared iodide of silver:—

Dissolve in a 3 or 4 oz. bottle containing 1 oz. water, 10 gr. nitrate of silver; and in another bottle or graduate containing a little water, dissolve 10 grains of iodide of potassium; pour this into the 10 grain solution of nitrate of silver, and a yellow substance (iodide of silver) will precipitate; fill the bottle with water, and let it settle; then pour off the water, leaving the yellow mass behind; again pour on it clean water, shake it, and let it settle as before, and pour off again; repeat this for about six changes of water.

Then it (the iodide of silver) is to be put into the bottle containing the 8 oz. water and 1800 gr. of nitrate of silver; shake it well, and it will nearly or quite all dissolve; pour this into the two-quart bottle, and shake well; it will be of a yellowish white tint, and should be filtered through asbestos or sponge, when it will become clear. When clear, test the solution with blue litmus-paper; if it turns it red, it is sufficiently acid; if it does not change it, add one or two drops of nitric acid, chemically pure; then test it again; if it does not change it, add one or two drops more, or just enough to change the paper to the slightest red.

A solution prepared in this proportion will, like others, improve by age. An old bath is considered far more valuable than one newly prepared. These remarks may appear to old photographic operators as of no importance, but they must bear in mind that there are hundreds just adopting this new process of picture taking.

This solution will work more satisfactorily than the one I formerly used. It will work quicker in the camera, and is equally durable.

Acknowledgment.—The following pages, under the head of Vocabulary of Photographic Chemicals, and treating upon the Chemicals used in Photography, are taken from the third edition of "Hardwich's Photographic Chemistry:"—

Vocabulary of Photographic Chemicals.

Acetic Acid.

Symbol, C{4}H{3}O{3} + HO. Atomic weight, 60.

Acetic acid is a product of the oxidation of alcohol. Spirituous liquids, when perfectly pure, are not affected by exposure to air; but if a portion of yeast, or nitrogenous organic matter of any kind, be added, it soon acts as a ferment, and causes the spirit to unite with oxygen derived from the atmosphere, and to become sour from formation of acetic acid or "vinegar."

Acetic acid is also produced on a large scale by heating wood in close vessels; a substance distils over which is acetic acid contaminated with empyreumatic and tarry matter; it is termed pyroligneous acid, and is much used in commerce.

The most concentrated acetic acid may be obtained by neutralizing common vinegar with carbonate of soda and crystallizing out the acetate of soda so formed; this acetate of soda is then distilled with sulphuric acid, which removes the soda and liberates acetic acid: the acetic acid being volatile, distils over, and may be condensed.

Properties of Acetic Acid.—The strongest acid contains only a single atom of water; it is sold under the name of "glacial acetic acid," so called from its property of solidifying at a moderately low temperature. At about 50° the crystals melt, and form a limpid liquid of pungent odor and a density nearly corresponding to that of water; the specific gravity of acetic acid, however, is no test of its real strength, which can only be estimated by analysis.

The commercial glacial acetic acid is often diluted with water, which may be suspected if it does not solidify during the cold winter months. Sulphurous and hydrochloric acids are also common impurities. They are injurious in photographic processes from their property of precipitating nitrate of silver. To detect them proceed as follows:—dissolve a small crystal of nitrate of silver in a few drops of water, and add to it about half a drachm of the glacial acid; the mixture should remain quite clear even when exposed to the light. Hydrochloric and sulphurous acids produce a white deposit of chloride or sulphite of silver; and if aldehyde or volatile tarry matter be present in the acetic acid, the mixture with nitrate of silver, although clear at first, becomes discolored by the action of light.

Many photographers employ a cheaper form of acetic acid, sold by druggists as "Beaufoy's" acid;[A] it should be of the strength of the acetic acid fortiss. of the London Pharmacopoeia, containing 30 per cent, real acid, and must be tested for sulphuric acid (see sulphuric acid), and also by mixing with nitrate of silver.

Acetate of Silver. (See Silver, Acetate of.)

Albumen.

Albumen is an organic principle, found both in the animal and vegetable kingdom. Its properties are best studied in the white of egg, which is a very pure form of albumen.

Albumen is capable of existing in two states; in one of which it is soluble, in the other insoluble in water. The aqueous solution of the soluble variety gives a slightly alkaline reaction to test-paper; it is somewhat thick and glutinous, but becomes more fluid on the addition of a small quantity of an alkali, such as potash or ammonia.

Soluble albumen may be converted into the insoluble form in the following ways:—

1. By the application of heat.—A moderately strong solution of albumen becomes opalescent and coagulates on being heated to about 150°, but a temperature of 212° is required if the liquid is very dilute. A layer of dried albumen cannot easily be coagulated by the mere application of heat.

2. By addition of strong acids.—Nitric acid coagulates albumen perfectly without the aid of heat. Acetic acid, however, acts differently, appearing to enter into combination with the albumen, and forming a compound soluble in warm water acidified by acetic acid.

3. By the action of metallic salts.—Many of the salts of the metals coagulate albumen very completely. Nitrate of silver does so; also the bichloride of mercury. Ammoniacal oxide of silver, however, does not coagulate albumen.

The white precipitate formed on mixing albumen with nitrate of silver is a chemical compound of the animal matter with protoxide of silver. This substance, which has been termed albuminate of silver, is soluble in ammonia and hyposulphite of soda; but after exposure to light, or heating in a current of hydrogen gas, it assumes a brick-red color, being probably reduced to the condition of a salt of the suboxide of silver. It is then almost insoluble in ammonia, but enough dissolves to tinge the liquid wine-red. The author is of opinion that the red coloration of solution of nitrate of silver employed in sensitizing the albumenized photographic paper is produced by the same compound, although often referred to the presence of sulphuret of silver.

Albumen also combines with lime and baryta; and chloride of barium has been recommended in positive printing upon albumenized paper, probably from this cause.

Chemical composition of albumen.—Albumen belongs to the nitrogenous class of organic substances. It also contains small quantities of sulphur and phosphorus.

Alcohol.

Symbol, C{4}H{6}O{2}. Atomic weight, 46.

Alcohol is obtained by the careful distillation of any spirituous or fermented liquor. If wine or beer be placed in a retort, and heat applied, the alcohol, being more volatile than water, rises first, and is condensed in an appropriate receiver; a portion of the vapor of water, however, passes over with the alcohol, and dilutes it to a certain extent, forming what is termed "spirits of wine." Much of this water may be removed by redistillation from carbonate of potash; but in order to render the alcohol thoroughly anhydrous, it is necessary to employ quick lime which possesses a still greater attraction for water. An equal weight of this powdered lime is mixed with strong alcohol of ·823, and the two are distilled together.

Properties of Alcohol.—Pure anhydrous alcohol is a limpid liquid, of an agreeable odor and pungent taste; sp. gr. at 60°, ·794. It absorbs vapor of water, and becomes diluted by exposure to damp air; boils at 173° Fahr. It has never been frozen.

Alcohol distilled from carbonate of potash has a specific gravity of ·815 to ·823, and contains 90 to 93 per cent, of real spirit.

The specific gravity of ordinary rectified spirits of wine is usually about ·840, and it contains 80 to 83 per cent, of absolute alcohol.

Ammonia.

Symbol, NH{3} or NH{4}O. Atomic weight, 17.

The liquid known by this name is an aqueous solution of the volatile gas ammonia. Ammoniacal gas contains 1 atom of nitrogen combined with three of hydrogen: these two elementary bodies exhibit no affinity for each other, but they can be made to unite under certain circumstances, and the result is ammonia.

Properties of Ammonia.—Ammoniacal gas is soluble in water to a large extent; the solution possessing those properties which are termed alkaline. Ammonia, however, differs from the other alkalies in one important particular—it is volatile: hence the original color of turmeric paper affected by ammonia is restored on the application of heat. Solution of ammonia absorbs carbonic acid rapidly from the air, and is converted into carbonate of ammonia; it should therefore be preserved in stoppered bottles. Besides carbonate, commercial ammonia often contains chloride of ammonium, recognized by the white precipitate given by nitrate of silver after acidifying with pure nitric acid.

The strength of commercial ammonia varies greatly; that sold for pharmaceutica purposes, under the name of liquor ammoniÆ, contains about 10 per cent, of real ammonia. The sp. gr. of aqueous ammonia diminishes with the proportion of ammonia present, the liquor ammoniÆ being usually about ·936.

Chemical Properties.—Ammonia, although forming a large class of salts, appears at first sight to contrast strongly by composition with the alkalies proper, such as potash and soda. Mineral bases generally are protoxides of metals, but ammonia consists simply of nitrogen and hydrogen united with oxygen. The following remarks may perhaps tend somewhat to elucidate the difficulty:—

Theory of Ammonium.—This theory supposes that a substance exists possessing the properties of a metal, but different from metallic bodies generally in being compound in structure: the formula assigned to it is NH{4}, 1 atom of nitrogen united with 4 of hydrogen. The hypothetical metal is termed "ammonium," and ammonia, associated with an atom of water, may be viewed as its oxide; for NH{3} + HO plainly equals NH{4}O. Thus, as potash is the oxide of potassium, so ammonia is the oxide of ammonium.

The composition of the salts of ammonia is on this view assimilated to those of the alkalies proper. Thus, sulphate of ammonia is a sulphate of the oxide of ammonium; muriate or hydrochlorate of ammonia is a chloride of ammonium, etc.

Ammonio-Nitrate of Silver.
(See Silver, Ammonio-Nitrate of.)

Aqua-Regia. (See Nitro-Hydrochloric Acid.)

Baryta, Nitrate of. (See Nitrate of Baryta.)

Bichloride of Mercury.
(See Mercury, Bichloride of.)

Bromine.

Symbol, Br. Atomic weight, 78.

This elementary substance is obtained from the uncrystallizable residuum of sea-water, termed bittern. It exists in the water in very minute proportion, combined with magnesium in the form of a soluble bromide of magnesium.

Properties.—Bromine is a deep reddish-brown liquid of a disagreeable odor, and fuming strongly at common temperatures; sparingly soluble in water (1 part in 23, Lowig), but more abundantly so in alcohol, and especially in ether. It is very heavy, having a specific gravity of 3·0.

Bromine is closely analogous to chlorine and iodine in its chemical properties. It stands on the list intermediately between the two; its affinities being stronger than those of iodine, but weaker than chlorine. (See Chlorine.)

It forms a large class of salts, of which the bromides of potassium, cadmium, and silver are the most familiar to photographers.

Bromide of Potassium.

Symbol, KBr. Atomic weight, 118.

Bromide of potassium is prepared by adding bromine to caustic potash, and heating the product, which is a mixture of bromide of potassium and bromate of potash, to redness, in order to drive off the oxygen from the latter salt. It crystallizes in anhydrous cubes, like the chloride, and iodide, of potassium; it is easily soluble in water, but more sparingly so in alcohol; it yields red fumes of bromine when acted upon by sulphuric acid.

Bromide of Silver. (See Silver, Bromide of.)

Carbonate of Soda.

Symbol, NaO CO{2} + 10 Aq.

This salt was formerly obtained from the ashes of seaweeds, but is now more economically manufactured on a large scale from common salt. The chloride of sodium is first converted into sulphate of soda, and afterwards the sulphate into carbonate of soda.

Properties.—The perfect crystals contain ten atoms of water, which are driven off by the application of heat, leaving a white powder—the anhydrous carbonate. Common washing soda is a neutral carbonate, contaminated to a certain extent with chloride of sodium and sulphate of soda. The carbonate used for effervescing draughts is either a bicarbonate with 1 atom of water, or a sesquicarbonate, containing about 40 per cent, of real alkali; it is therefore nearly double as strong as the washing carbonate, which contains about 22 per cent, of soda. Carbonate of soda is soluble in twice its weight of water at 60°, the solution being strongly alkaline.

Carbonate of Potash. (See Potash, Carbonate of.)

Caseine. (See Milk.)

Charcoal, Animal.

Animal charcoal is obtained by heating animal substances, such as bones, dried blood, horns, etc., to redness, in close vessels, until all volatile empyreumatic matters have been driven off, and a residue of carbon remains. When prepared from bones it contains a large quantity of inorganic matter in the shape of carbonate and phosphate of lime, the former of which produces alkalinity in reacting upon nitrate of silver. Animal charcoal is freed from these earthy salts by repeated digestion in hydrochloric acid; but unless very carefully washed it is apt to retain an acid reaction, and so to liberate free nitric acid when added to solution of nitrate of silver.

Properties.—Animal charcoal, when pure, consists solely of carbon, and burns away in the air without leaving any residue: it is remarkable for its property of decolorizing solutions; the organic coloring substance being separated, but not actually destroyed, as it is by chlorine employed as a bleaching agent. This power of absorbing coloring matter is not possessed in an equal degree by all varieties of charcoal, but is in great measure peculiar to those derived from the animal kingdom.

China Clay or Kaolin.

This is prepared, by careful levigation, from mouldering granite and other disintegrated felspathic rocks. It consists of the silicate of alumina,—that is, of silicic acid or flint, which is an oxide of silicon, united with the base alumina (oxide of aluminum). Kaolin is perfectly insoluble in water and acids, and produces no decomposition in solution of nitrate of silver. It is employed by photographers to decolorize solutions of nitrate of silver which have become brown from the action of albumen or other organic matters.

Chlorine.

Symbol, Cl. Atomic weight, 36.

Chlorine is a chemical element found abundantly in nature, combined with metallic sodium in the form of chloride of sodium, or sea-salt.

Preparation.—By distilling common salt with sulphuric acid, sulphate of soda and hydrochloric acid are formed. Hydrochloric acid contains chlorine combined with hydrogen; by the action of nascent oxygen (see oxygen), the hydrogen may be removed in the form of water, and the chlorine left alone.

Properties.—Chlorine is a greenish-yellow gas, of a pungent and suffocating odor; soluble to a considerable extent in water, the solution possessing the odor and color of the gas. It is nearly 2½ times as heavy as a corresponding bulk of atmospheric air.

Chemical Properties.—Chlorine belongs to a small natural group of elements which contains also bromine, iodine, and fluorine. They are characterized by having a strong affinity for hydrogen, and also for the metals, but are comparatively indifferent to oxygen. Many metallic substances actually undergo combustion when projected into an atmosphere of chlorine, the union between the two taking place with extreme violence. The characteristic bleaching properties of chlorine gas are explained in the same manner:—Hydrogen is removed from the organic substance, and in that way the structure is broken up and the color destroyed.

Chlorine is more powerful in its affinities than either bromine or iodine. The salts formed by these three elements are closely analogous in composition and often in properties. Those of the alkalies, alkaline earths, and many of the metals are soluble in water, but the silver salts are insoluble; the lead salts sparingly so.

The combinations of chlorine, bromine, iodine, and fluorine, with hydrogen, are acids, and neutralize alkalies in the usual manner, with formation of alkaline chloride and water.

The test by which the presence of chlorine is detected, either free or in combination with bases, is nitrate of silver; it gives a white curdy precipitate of chloride of silver, insoluble in nitric acid, but soluble in ammonia. The solution of nitrate of silver employed as the test must not contain iodide of silver, as this compound is precipitated by dilution.

Chloride of Ammonium.

Symbol, NH{4}Cl. Atomic weight, 54.

This salt, also known as muriate or hydrochlorate of ammonia, occurs in commerce in the form of colorless and translucent masses, which are procured by sublimation, the dry salt being volatile when strongly heated. It dissolves in an equal weight of boiling, or in three parts of cold water. It contains more chlorine in proportion to the weight used than chloride of sodium, the atomic weights of the two being as 54 to 60.

Chloride of Barium.

Symbol, BaCl+2HO. Atomic weight, 123.

Barium is a metallic element, very closely allied to calcium, the elementary basis of lime. The chloride of barium is commonly employed as a test for sulphuric acid, with which it forms an insoluble precipitate of sulphate of baryta. It is also said to affect the color of the photographic image when used in preparing positive paper; which may possibly be due to a chemical combination of baryta with albumen: but it must be remembered that this chloride, from its high atomic weight, contains less chlorine than the alkaline chlorides.

Properties of Chloride of Barium.—Chloride of barium occurs in the form of white crystals, soluble in about two parts of water, at common temperature. These crystals contain two atoms of water of crystallization, which are expelled at 212°, leaving the anhydrous chloride.

Chloride of Gold. (See Gold, Chloride of.)

Chloride of Sodium.

Symbol, NaCl. Atomic weight, 60.

Common salt exists abundantly in nature, both in the form of solid rock-salt and dissolved in the waters of the ocean.

Properties of the pure Salt.—Fusible without decomposition at low redness, but sublimes at higher temperatures; the melted salt concretes into a hard white mass on cooling. Nearly insoluble in absolute alcohol, but dissolves in minute quantity in rectified spirit. Soluble in three parts of water, both hot and cold. Crystallizes in cubes, which are anhydrous.

Impurities of Common Salt.—Table salt often contains large quantities of the chlorides of magnesium and calcium, which, being deliquescent, produce a dampness by absorption of atmospheric moisture: sulphate of soda is also commonly present. The salt may be purified by repeated recrystallization, but it is more simple to prepare the pure compound directly, by neutralizing hydrochloric acid with carbonate of soda.

Chloride of Silver. (See Silver, Chloride of.)

Citric Acid.

This acid is found abundantly in lemon-juice and in lime-juice. It occurs in commerce in the form of large crystals, which are soluble in less than their own weight of water at 60°.

Commercial citric acid is sometimes mixed with tartaric acid. The adulteration may be discovered by making a concentrated solution of the acid and adding acetate of potash; crystals of bitartrate of potash will separate if tartaric acid be present.

Citric acid is tribasic. It forms with silver a white insoluble salt, containing 3 atoms of oxide of silver to 1 atom of citric acid. If the citrate of silver be heated in a current of hydrogen gas, a part of the acid is liberated and the salt is reduced to a citrate of suboxide of silver; which is of a red color. The action of white light in reddening citrate of silver is shown by the author to be of a similar nature.

Cyanide of Potassium.

Symbol, K, C{2}N, or KCy. Atomic weight, 66.

This salt is a compound of cyanogen gas with the metal potassium. Cyanogen is not an elementary body, like chlorine or iodine, but consists of carbon and nitrogen united in a peculiar manner. Although a compound substance, it reacts in the manner of an element, and is therefore (like ammonium, previously described) an exception to the usual laws of chemistry. Many other bodies of a similar character are known.

Ether.

Symbol, C{4}H{5}O. Atomic weight, 37.

Ether is obtained by distilling a mixture of sulphuric acid and alcohol. If the formula of alcohol (C{4}H{6}O{2}) be compared with that of ether, it will be seen to differ from it in the possession of an additional atom of hydrogen and of oxygen: in the reaction, the sulphuric acid removes these elements in the form of water, and by so doing converts one atom of alcohol into an atom of ether. The term sulphuric applied to the commercial ether has reference only to the manner of its formation.

Properties of Ether.—It is neither acid nor alkaline to test-paper. Specific gravity, at 60°, about ·720. Boils at 98° Fahrenheit. The vapor is exceedingly dense, and may be seen passing off from the liquid and falling to the ground: hence the danger of pouring ether from one bottle to another if a flame be near at hand.

Ether does not mix with water in all proportions; if the two are shaken together, after a short time the former rises and floats upon the surface. In this way a mixture of ether and alcohol may be purified to some extent, as in the common process of washing ether. The water employed however always retains a certain portion of ether (about a tenth part of its bulk), and acquires a strong ethereal odor; washed ether also contains water in small quantity.

Bromine and iodine are both soluble in ether, and gradually react upon and decompose it.

The strong alkalies, such as potash and soda, also decompose ether slightly after a time, but not immediately. Exposed to air and light, ether is oxidized and acquires a peculiar odor.

Ether dissolves fatty and resinous substances readily, but inorganic salts are mostly insoluble in this fluid. Hence it is that iodide of potassium and other substances dissolved in alcohol are precipitated to a certain extent by the addition of ether.

Fluoride of Potassium.

Symbol, KF. Atomic weight, 59.

Preparation.—Fluoride of potassium is formed by saturating hydrofluoric acid with potash, and evaporating to dryness in a platinum vessel. Hydrofluoric acid contains fluorine combined with hydrogen; it is a powerfully acid and corrosive liquid, formed by decomposing flour spar, which is a fluoride of calcium, with strong sulphuric acid; the action which takes place being precisely analogous to that involved in the preparation of hydrochloric acid.

Properties.—A deliquescent salt, occurring in small and imperfect crystals. Very soluble in water: the solution acting upon glass in the same manner as hydrofluoric acid.

Formic Acid.

Symbol, C{2}HO{3}. Atomic weight, 37.

This substance was originally discovered in the red ant (Formica rufa), but it is prepared on a large scale by distilling starch with binoxide of manganese and sulphuric acid.

Properties.—The strength of commercial formic acid is uncertain, but it is always more or less dilute. The strongest acid, as obtained by distilling formiate of soda with sulphuric acid, is a fuming liquid with a pungent odor, and containing only one atom of water: it inflames the skin in the same manner as the sting of the ant.

Formic acid reduces the oxides of gold, silver, and mercury, to the metallic state, and is itself oxidized into carbonic acid. The alkaline formiates also possess the same properties.

Gelatine.

Symbol, C{13}H{10}O{5}N{2}. Atomic weight, 156.

This is an organic substance somewhat analogous to albumen, but differing from it in properties. It is obtained by subjecting bones, hoofs, horns, calves' feet, etc., to the action of boiling water. The jelly formed on cooling is termed size, or when dried or cut into slices, glue. Gelatine, as it is sold in the shops, is a pure form of glue. Isinglass is gelatine prepared, chiefly in Russia, from the air-bladders of certain species of sturgeon.

Properties of Gelatine.—Gelatine softens and swells up in cold water, but does not dissolve until heated: the hot solution, on cooling, forms a tremulous jelly. One ounce f cold water will retain about three grains of isinglass without gelatinizing; but much depends upon the temperature, a few degrees greatly affecting the result.

Gelatine forms no compound with oxide of silver analogous to the albuminate of silver; which fact explains the difference in the photographic properties of albumen and gelatine.

Glycerine.

Fatty bodies are resolved by treatment with an alkali into an acid—which combines with the alkali, forming a soap,—and glycerine, remaining in solution.

Pure glycerine, as obtained by Price's patent process of distillation, is a viscid liquid of sp. gr. about 1·23; miscible in all proportions with water and alcohol. It is peculiarly a neutral substance, exhibiting no tendency to combine with acids or bases. It has little or no action upon nitrate of silver in the dark, and reduces it very slowly even when exposed to light.

Gold, Chloride of.

Symbol, AuCl{3}. Atomic weight, 303.

This salt is formed by dissolving pure metallic gold in nitro-hydrochloric acid, and evaporating at a gentle heat. The solution affords deliquescent crystals of a deep orange color.

Chloride of gold, in a state fit for photographic use may easily be obtained by the following process:—Place a half-sovereign in any convenient vessel, and pour on it half a drachm of nitric acid mixed with two and a half drachms of hydrochloric acid and three drachms of water; digest by a gentle heat, but do not boil the acid, or much of the chlorine will be driven off in the form of gas. At the expiration of a few hours add fresh aqua-regia in quantity the same as at first, which will probably complete the solution, but if not, repeat the process a third time.

Lastly, neutralize the liquid by adding carbonate of soda until all effervescence ceases, and a green precipitate forms; this is carbonate of copper, which must be allowed several hours to separate thoroughly. The solution then contains chloride of gold in a neutral state, and free from copper and silver, with which the metallic gold is alloyed in the standard coin of the realm.

The weight of a half-sovereign is about 61 grains, of which 56 grains are pure gold. This is equivalent to 86 grains of chloride of gold, which will therefore be the quantity contained in the solution.

The following process for preparing chloride of gold is more perfect than the last:—dissolve the gold coin in aqua-regia as before; then boil with excess of hydrochloric acid to destroy the nitric acid, dilute largely with distilled water, and add a filtered aqueous solution of common sulphate of iron (6 parts in 1 part of gold); collect the precipitated gold, which is now free from copper; re-dissolve in aqua-regia, and evaporate to dryness on a water bath.

Avoid using ammonia to neutralize chloride of gold, as it would be liable to occasion a deposit of "fulminating gold," the properties of which are described immediately following.

Properties of Chloride of Gold.—As sold in commerce it usually contains excess of hydrochloric acid, and is then of a bright yellow color; but when neutral and somewhat concentrated it is dark red (Leo ruber of the alchemists). It gives no precipitate with carbonate of soda, unless heat be applied; the free hydrochloric acid present forms, with the alkali, chloride of sodium, which unites with the chloride of gold, and produces a double salt, chloride of gold and sodium, soluble in water.

Chloride of gold is decomposed with precipitation of metallic gold by charcoal, sulphurous acid, and many of the vegetable acids; also by protosulphate and protonitrate of iron. It tinges the cuticle of an indelible purple tint. It is soluble in alcohol and in ether.

Gold, Fulminating.

This is a yellowish-brown substance, precipitated on adding ammonia to a strong solution of chloride of gold.

It may be dried carefully at 212°, but explodes violently on being heated suddenly about to 290°. Friction also causes it to explode when dry; but the moist powder may be rubbed or handled without danger. It is decomposed by sulphuretted hydrogen.

Fulminating gold is probably an aurate of ammonia, containing 2 atoms of ammonia to 1 atom of peroxide of gold.

Gold, Hyposulphite of.

Symbol, AuO S{2}O{2}. Atomic Weight, 253.

Hyposulphite of gold is produced by the reaction of chloride of gold upon hyposulphite of soda.

The salt sold in commerce as sel d'or is a double hyposulphite of gold and soda, containing one atom of the former salt to three of the latter, with four atoms of water of crystallization. It is formed by adding one part of chloride of gold, in solution, to three parts of hyposulphite of soda, and precipitating the resulting salt by alcohol; the chloride of gold must be added to the hyposulphite of soda, and not the soda salt to the gold.

Properties.—Hyposulphite of gold is unstable and cannot exist in an isolated state, quickly passing into sulphur, sulphuric acid, and metallic gold. When combined with excess of hyposulphite of soda in the form of sel d'or, it is more permanent.

Sel d'or occurs crystallized in fine needles, which are very soluble in water. The commercial article is often impure, containing little else than hyposulphite of soda, with a trace of gold. It may be analyzed by adding a few drops of strong nitric acid (free from chlorine) diluting with water, and afterwards collecting and igniting the yellow powder, which is metallic gold.

Grape Sugar.

Symbol, C{24}H{28}O{28}. Atomic weight, 366.

This modification of sugar, often termed granular sugar, or glucose, exists abundantly in the juice of grapes, and in many other varieties of fruit. It forms the saccharine concretion found in honey, raisins, dried figs, etc. It may be produced artificially by the action of fermenting principles, and of dilute mineral acids, upon starch.

Properties.—Grape sugar crystallizes slowly and with difficulty from a concentrated aqueous solution, in small hemispherical nodules, which are hard, and feel gritty between the teeth. It is much less sweet to the taste than cane sugar, and not so soluble in Water (1 part dissolves in 1½ of cold water). Grape sugar tends to absorb oxygen, and hence it possesses the property of decomposing the salts of the noble metals, and reducing them by degrees to the metallic state, even without the aid of lights The action however in the case of nitrate of silver is slow, unless the temperature be somewhat elevated. Cane sugar does not possess these properties to an equal extent, and hence it is readily distinguished from the other variety.

Honey.

This substance contains two distinct kinds of sugar, grape sugar, and an uncrystallizable substance analogous to, or identical with, the treacle found associated with common sugar in the cane juice. The agreeable taste of honey probably depends upon the latter, but its reducing power on metallic oxides is due to the former. Pure grape sugar can readily be obtained from inspissated honey, by treating it with alcohol, which dissolves out the syrup, but leaves the crystalline portion.

Hydrochloric; Acid.

Symbol, HCl. Atomic weight, 37.

Hydrochloric acid is a volatile gas, Which may be liberated from the salts termed chlorides by the action of sulphuric acid. The acid, by its superior affinities, removes the base; thus,—

NaCl + HO SO{3} = NaO SO{3} + HCl.

Properties.—Abundantly soluble in water, forming the liquid hydrochloric or muriatic acid of commerce. The most concentrated solution of hydrochloric acid has a sp. gr. 1·2, and contains about 40 per cent, of gas; that commonly sold is somewhat weaker, sp; gr. 1·14 = 28 per cent. real acid.

Pure hydrochloric acid is colorless, and fumes in the air. The yellow color of the commercial acid depends upon the presence of traces of perchloride of iron or organic matter; commercial muriatic acid also often contains a portion of free chlorine and of sulphuric acid.

Hydriodic Acid.

Symbol, HI. Atomic weight, 127.

This is a gaseous compound of hydrogen and iodine, corresponding in composition to the hydrochloric acid. It cannot, however, from its instability, be obtained in the same manner, since, on distilling an iodide with sulphuric acid, the hydriodic acid first formed is subsequently decomposed into iodine and hydrogen. An aqueous solution of hydriodic acid is easily prepared by adding iodine to water containing sulphuretted hydrogen gas; a decomposition takes place, and sulphur is set free; thus: HS + I = HI + S.

Properties.—Hydriodic acid is very soluble in water, yielding a strongly acid liquid. The solution, colorless at first, soon becomes brown from decomposition, and liberation of free iodine. It may be restored to its original condition by adding solution of sulphuretted hydrogen.

Hydrosulphuric Acid.

Symbol, HS. Atomic weighty 17.

This substance, also known as sulphuretted hydrogen, is a gaseous compound of sulphur and hydrogen, analogous in composition to hydrochloric and hydriodic acids. It is usually prepared by the action of dilute sulphuric acid upon sulphuret of iron, the decomposition being similar to that involved in the preparation of the hydrogen acids generally:—

FeS + HO SO{3} = FeO SO{3} + HS.

Properties.—Cold water absorbs three times its bulk of hydrosulphuric acid, and acquires the peculiar putrid odor and poisonous qualities of the gas. The solution is faintly acid to test-paper, and becomes opalescent on keeping, from gradual separation of sulphur. It is decomposed by nitric acid, and also by chlorine and iodine. It precipitates silver from its solutions, in the form of black sulphuret of silver; also copper, mercury, lead, etc.; but iron and other metals of that class are not affected, if the liquid contains free acid. Hydrosulphuric acid is constantly employed in the chemical laboratory for these and other purposes.

Hydrosulphate of Ammonia.

Symbol, NH{4}S HS. Atomic weight, 51.

The liquid known by this name, and formed by passing sulphuretted hydrogen gas into ammonia, is a double sulphuret of hydrogen and ammonium. In the preparation, the passage of the gas is to be continued until the solution gives no precipitate with sulphate of magnesia and smells strongly of hydrosulphuric acid.

Properties,—Colorless at first, but afterwards changes to yellow, from liberation and subsequent solution of sulphur. Becomes milky on the addition of any acid. Precipitates, in the form of sulphuret, all the metals which are affected by sulphuretted hydrogen; and, in addition, those of the class to which iron, zinc, and manganese, belong.

Hydrosulphate of ammonia is employed in photography to darken the negative image, and also in the preparation of iodide of ammonium; the separation of silver from hyposulphite solutions, etc.

Hyposulphite of Soda.

Symbol, NaO S{2}H{2} + 5 HO. Atomic weight, 125.

The hyposulphite of soda commonly employed by photographers is a neutral combination of hyposulphurous acid and the alkali soda. It is selected as being more economical in preparation than any other hyposulphite adapted for fixing.

Hyposulphite of soda occurs in the form of large translucent groups of crystals, which include five atoms of water. These crystals are soluble in water almost to any extent, the solution being attended with the production of cold; they have a nauseous and bitter taste.

Hyposulphite of Gold. (See Gold, Hyposulphite of.)

Hyposulphite of Silver. (See Silver, Hyposulphite of.)

Iceland Moss.

Cetraria Islandica.—A species of lichen found in Iceland and the mountainous parts of Europe; when boiled in water, it first swells up, and then yields a substance which gelatinizes on cooling.

It contains lichen starch; a bitter principle soluble in alcohol, termed "cetrarine;" and common starch; traces of gallic acid and bitartrate of potash are also present.

Iodine.

Symbol, I. Atomic weight, 126.

Iodine is chiefly prepared at Glasgow, from kelp, which is the fused ash obtained by burning seaweeds. The waters of the ocean contain minute quantities of the iodides of sodium and magnesium, which are separated and stored up by the growing tissues of the marine plant.

In the preparation, the mother-liquor of kelp is evaporated to dryness and distilled with sulphuric acid; the hydriodic acid first liberated is decomposed by the high temperature, and fumes of iodine condense in the form of opaque crystals.

Properties.—Iodine has a bluish-black color and metallic lustre; it stains the skin yellow, and has a pungent smell, like diluted chlorine. It is extremely volatile when moist, boils at 350°, and produces dense violet-colored fumes, which condense in brilliant plates. Specific gravity 4·946. Iodine is very sparingly soluble in water, 1 part requiring 7000 parts for perfect solution: even this minute quantity however tinges the liquid of a brown color. Alcohol and ether dissolve it more abundantly, forming dark-brown solutions. Iodine also dissolves freely in solutions of the alkaline iodides, such as the iodide of potassium, of sodium, and of ammonium.

Chemical Properties.—Iodine belongs to the chlorine group of elements, characterized by forming acids with hydrogen, and combining extensively with the metals (see chlorine). They are however comparatively indifferent to oxygen, and also to each other. The iodides of the alkalies and alkaline earths are soluble in water; also those of iron, zinc, cadmium, etc. The iodides of lead, silver, and mercury are nearly or quite insoluble.

Iodine possesses the property of forming a compound of a deep blue color with starch. In using this as a test, it is necessary first to liberate the iodine (if in combination), by means of chlorine, or nitric acid saturated with peroxide of nitrogen. The presence of alcohol or ether interferes to a certain extent with the result.

Iodide of Ammonium.

Symbol, NH{4}I. Atomic weight, 144.

This salt may be prepared by adding carbonate of ammonia to iodide of iron, but more easily by the following process:—A strong solution of hydrosulphate of ammonia is first made, by passing sulphuretted hydrogen gas into liquor ammoniÆ To this liquid iodine is added until the whole of the sulphuret of ammonium has been converted into iodide. When this point is reached, the solution at once colors brown from solution of free iodine. On the first addition of the iodine, an escape of sulphuretted hydrogen gas and a dense deposit of sulphur take place. After the decomposition of the hydrosulphate of ammonia is complete, a portion of hydriodic acid—formed by the mutual reaction of sulphuretted hydrogen and iodine—attacks any carbonate of ammonia which may be present, and causes an effervescence. The effervescence being over, the liquid is still acid to test-paper, from excess of hydriodic acid; it is to be cautiously neutralized with ammonia, and evaporated by the heat of a water-bath to the crystallizing point.

The crystals should be thoroughly dried over a dish of sulphuric acid, and then sealed in small tubes containing each about half a drachm of the salt; by this means it will be preserved colorless.

Iodide of ammonium is very soluble in alcohol, but it is not advisable to keep it in solution, from the rapidity with which it decomposes and becomes brown.

The most common impurity of commercial iodide of ammonium is sulphate of ammonia; it is detected by its sparing insolubility in alcohol.

Iodide of Cadmium.

Symbol, CdI. Atomic weight, 182.

This salt is formed by heating filings of metallic cadmium with iodine, or by mixing the two together with addition of water. It is useful in iodizing collodion intended for keeping, since it does not become brown from liberation of free iodine with the same rapidity as the alkaline iodides.

Iodide of cadmium is very soluble both in alcohol and water; the solution yielding on evaporation large six-sided tables of a pearly lustre, which are permanent in the air. The crystalline form of this salt is a sufficient criterion of its purity.

Iodide of Iron.

Symbol, FeI. Atomic weight, 154.

Iodide of iron, in a fit state for photographic use, is easily obtained by dissolving a drachm of iodine in an ounce of proof spirit—that is, a mixture of equal bulks of spirits of wine and water—and adding an excess of iron filings. After a few hours, a green solution is obtained without the aid of heat. The presence of metallic iron in excess prevents the liberation of iodine and deposit of peroxide of iron which would otherwise speedily occur. It is very soluble in water and alcohol, but the solution rapidly absorbs oxygen and deposits peroxide of iron; hence the importance of preserving it in contact with metallic iron, with which the separated iodine may recombine. By very careful evaporation, hydrated crystals of protoiodide may be obtained, but the composition of the solid salt usually sold under that name cannot be depended on.

The periodide of iron, corresponding to the perchloride, has not been examined, and it is doubtful if any such compound exists.

Iodide of Potassium.

Symbol, KI. Atomic weight, 166.

This salt is usually formed by dissolving iodine in solution of potash until it begins to acquire a brown color; a mixture of iodide of potassium and iodate of potash (KO IO{5}) is thus formed; but by evaporation and heating to redness, the latter salt parts with its oxygen, and is converted into iodide of potassium.

Properties.—It forms cubic and prismatic crystals, which should be hard, and very slightly or not at all deliquescent. Soluble in less than an equal weight of water at 60°; it is also soluble in alcohol, but not in ether. The proportion of iodide of potassium contained in a saturated alcoholic solution, varies with the strength of the spirit,—with common spirits of wine, sp. gr. ·836, it would be about 8 grains to the drachm; with alcohol rectified from carbonate of potash, sp. gr. ·823, 4 or 5 grains: with absolute alcohol, 1 to 2 grains. The solution of iodide of potassium is instantly colored brown by free chlorine; also very rapidly by peroxide of nitrogen; ordinary acids, however, act less quickly, hydriodic acid being first formed, and subsequently decomposing spontaneously.

Iodide of potassium, as sold in the shops, is often contaminated with various impurities. The first and most remarkable is carbonate of potash. When a sample of iodide of potassium contains much carbonate of potash, it forms small and imperfect crystals, which are strongly alkaline to test-paper, and become moist on exposure to the air, from the deliquescent nature of the alkaline carbonate. Sulphate of potash is also a common impurity; it may be detected by chloride of barium.

Chloride of potassium is another impurity; it is detected as follows:—Precipitate the salt by an equal weight of nitrate of silver, and treat the yellow mass with solution of ammonia; if any chloride of silver is present, it dissolves in the ammonia, and after nitration is re-precipitated in white curds by the addition of an excess of pure nitric acid. If the nitric acid employed is not pure, but contains traces of free chlorine, the iodide of silver must be well washed with distilled water before treating it with ammonia, or the excess of free nitrate of silver dissolving in the ammonia would, on neutralizing, produce chloride of silver, and so cause an error.

Iodide of potash is a fourth impurity often found in iodide of potassium: to detect it, add a drop of dilute sulphuric acid, or a crystal of citric acid, to the solution of the iodide; when, if much iodate be present, the liquid will become yellow from liberation of free iodine. The rationale of this reaction is as follows:—The sulphuric acid unites with the base of the salt, and liberates hydriodic acid (HI), a colorless compound; but if iodic acid (IO{5}) be also present, it decomposes the hydriodic acid first formed, oxidizing the hydrogen into water (HO), and setting free the iodine. The immediate production of a yellow color on adding a weak acid to aqueous solution of iodide of potassium is, therefore, a proof of the presence of an iodate. As iodate of potash is thought to render collodion insensitive (?), this point should be attended to.

Iodide of potassium may be rendered very pure by recrystallizing from spirit, or by dissolving in strong alcohol of sp. gr. ·823, in which sulphate, carbonate, and iodate of potash are insoluble. The proportion of iodide of potassium contained in saturated alcoholic solutions varies with the strength of the spirit.

Solution of chloride of barium is commonly used to detect impurities in iodide of potassium; it forms a white precipitate if carbonate, iodate, or sulphate be present. In the two former cases the precipitate dissolves on the addition of pure dilute nitric acid, but in the latter it is insoluble. The commercial iodide, however, is rarely so pure as to remain quite clear on the addition of chloride of barium, a mere opalescence, therefore, may be disregarded.

Iodide of Silver. (See Silver, Iodide of.)

Iron, Protosulphate of.

Symbol, FeO SO{3} + 7 HO. Atomic weight, 139.

This salt, often termed copperas or green vitriol, is a most abundant substance, and used for a variety of purposes in the arts. Commercial sulphate of iron, however, being prepared on a large scale, requires recrystallization to render it sufficiently pure for photographic purposes.

Pure sulphate of iron occurs in the form of large, transparent prismatic crystals, of a delicate green color: by exposure to the air they gradually absorb oxygen and become rusty on the surface. Solution of sulphate of iron, colorless at first, afterwards changes to a red tint, and deposits a brown powder; this powder is a basic persulphate of iron, that is, a persulphate containing an excess of the oxide or base. By the addition of sulphuric or acetic acid to the solution, the formation of a deposit is prevented, the brown powder being soluble in acid liquids.

The crystals of sulphate of iron include a large quantity of water of crystallization, a part of which they lose by exposure to dry air. By a higher temperature, the salt may be rendered perfectly anhydrous, in which state it forms a white powder.

Aqueous solution of sulphate of iron absorbs the binoxide of nitrogen, acquiring a deep olive-brown color: as this gaseous binoxide is itself a reducing agent, the liquid so formed has been proposed as a more energetic developer than the sulphate of iron alone.

Iron, Protonitrate of.

Symbol, FeO NO{5} + 7 HO. Atomic weight, 153.

This salt, by careful evaporation in vacuo over sulphuric acid, forms transparent crystals, of a light green color, and containing 7 atoms of water, like the protosulphate. It is exceedingly unstable, and soon becomes red from decomposition, unless preserved from contact with air.

The following process is commonly followed for preparing protonitrate of iron:—

Take of nitrate of baryta 300 grains; powder and dissolve by the aid of heat in three ounces of water; then throw in, by degrees, with constant stirring, crystallized sulphate of iron, powdered, 320 grains. Continue to stir for about five or ten minutes. Allow to cool, and filter from the white deposit, which is the insoluble sulphate of baryta.

In place of nitrate of baryta, the nitrate of lead may be used (sulphate of lead being an insoluble salt), but the quantity required will be different. The atomic weights of nitrate of baryta and nitrate of lead are as 131 to 166; consequently 300 grains of the former are equivalent to 380 grains of the latter.

Iron, Perchloride of.

Symbol, Fe{2}Cl{3}. Atomic weight, 164.

There are two chlorides of iron, corresponding in composition to the protoxide and the sesquioxide respectively. The protochloride is very soluble in water, forming a green solution, which precipitates a dirty white protoxide on the addition of an alkali. The perchloride, on the other hand, is dark brown, and gives a foxy-red precipitate with alkalies.

Properties.—Perchloride of iron may be obtained in the solid form by heating iron wire in excess of chlorine; it condenses in the shape of brilliant and iridescent brown crystals, which are volatile, and dissolve in water, the solution being acid to test-paper. It is also soluble in alcohol, forming the tinctura ferri sesquichloridi of the Pharmacopoeia. Commercial perchloride of iron ordinarily contains an excess of hydrochloric acid.

Litmus.

Litmus is a vegetable substance, prepared from various lichens, which are principally collected on rocks adjoining the sea. The coloring matter is extracted by a peculiar process, and afterwards made up into a paste with chalk, plaster of Paris, &c.

Litmus occurs in commerce in the form of small cubes, of a fine violet color. In using it for the preparation of test-papers, it is digested in hot water, and sheets of porous paper are soaked in the blue liquid so formed. The red papers are prepared at first in the same manner, but afterwards placed in water which has been rendered faintly acid with sulphuric or hydrochloric acid.

Mercury, Bichloride of.

Symbol, HgCl{2}. Atomic weight, 274.

This salt, also called corrosive sublimate, and sometimes chloride of mercury (the atomic weight of mercury being halved), may be formed by heating mercury in excess of chlorine, or, more economically, by subliming a mixture of persulphate of mercury and chloride of sodium.

Properties.—a very corrosive and poisonous salt, usually sold in semi-transparent, crystalline masses, or in the state of powder. Soluble in 16 parts of cold, and in 3 of hot water; more abundantly so in alcohol, and also in ether. The solubility in water may be increased almost to any extent by the addition of free hydrochloric acid.

The protochloride of mercury is an insoluble white powder, commonly known under the name of calomel.

Milk.

The milk of herbivorous animals contains three principal constituents—fatty matter, caseine, and sugar; in addition to these, small quantities of the chloride of potassium, and of phosphates of lime and magnesia, are present.

The fatty matter is contained in small cells, and forms the greater part of the cream which rises to the surface of the milk on standing. Hence skimmed milk is to be preferred for photographic use.

The second constituent, caseine, is an organic principle somewhat analogous to albumen in composition and properties. Its aqueous solution however does not, like albumen, coagulate on boiling, unless an acid be present, which probably removes a small portion of alkali with which the caseine was previously combined. The substance termed "rennet," which is the dried stomach of the calf, possesses the property of coagulating caseine, but the exact mode of its action is unknown. Sherry wine is also employed to curdle milk; but brandy and other spirituous liquids, when free from acid and astringent matter, have no effect.

In all these cases a proportion of the caseine usually remains in a soluble form in the whey; but when the milk is coagulated by the addition of acids, the quantity so left is very small, and hence the use of the rennet is to be preferred, since the presence of caseine facilitates the reduction of the sensitive silver salts.

Caseine combines with oxide of silver in the same manner as albumen, forming a white coagulum, which becomes brick-red on exposure to light.

Sugar of milk, the third principal constituent, differs from both cane and grape sugar; it may be obtained by evaporating whey until crystallization begins to take place. It is hard and gritty, and only slightly sweet; slowly soluble, without forming a syrup, in about two and a half parts of boiling, and six of cold water. It does not ferment and form alcohol on the addition of yeast, like grape sugar, but by the action of decomposing animal matter is converted into lactic acid.

When skimmed milk is exposed to the air for some hours it gradually becomes sour, from lactic acid formed in this way; and if then heated to ebullition, the caseine coagulates very perfectly.

Nitric Acid.

Symbol, NO{5}. Atomic weight, 54.

Nitric acid, or aqua-fortis, is prepared by adding sulphuric acid to nitrate of potash, and distilling the mixture in a retort. Sulphate of potash and free nitric acid are formed, the latter of which, being volatile, distils over in combination with one atom of water previously united with sulphuric acid.

Properties.—Anhydrous nitric acid is a solid substance, white and crystalline, but it cannot be prepared except by an expensive and complicated process.

The concentrated liquid nitric acid contains 1 atom of water, and has a sp. gr. of about 1·5: if perfectly pure it is colorless, but usually it has a slight yellow tint, from partial decomposition into peroxide of nitrogen: it fumes strongly in the air.

The strength of commercial nitric acid is subject to much variation. An acid of sp. gr. 1·42, containing about 4 atoms of water, is commonly met with. If the specific gravity is much lower than this (less than 1·36), it will scarcely be adapted for the preparation of peroxyline. The yellow nitrous acid, so called, is a strong nitric acid partially saturated with the brown vapors of peroxide of nitrogen; it has a high specific gravity, but this is somewhat deceptive, being caused in part by the presence of the peroxide. On mixing with sulphuric acid the color disappears, a compound being formed which has been termed a sulphate of nitrous acid.

Chemical properties.—Nitric acid is a powerful oxidizing agent; it dissolves all the common metals, with the exception of gold and platinum. Animal substances, such as the cuticle, nails, etc., are tinged of a permanent yellow color, and deeply corroded by a prolonged application. Nitric acid forms a numerous class of salts, all of which are soluble in water. Hence its presence cannot be determined by any precipitating re-agent, in the same manner as that of hydrochloric and sulphuric acid.

Impurities of Commercial Nitric Acid.—These are principally chlorine and sulphuric acid; also peroxide of nitrogen, which tinges the acid yellow, as already described. Chlorine is detected by diluting the acid with an equal bulk of distilled water, and adding a few drops of nitrate of silver,—a milkiness, which is chloride of silver in suspension, indicates the presence of chlorine. In testing for sulphuric acid, dilute the nitric acid as before, and drop in a single drop of solution of chloride of barium; if sulphuric acid be present, an insoluble precipitate of sulphate of baryta will be formed.

Nitrous Acid. (See Silver, Nitrate of.)

Nitrate of Potash.

Symbol, KO NO{5}. Atomic weight, 102.

This salt, also termed nitre or saltpetre, is an abundant natural product, found effloresced upon the soil in certain parts of the East Indies. It is also produced artificially in what are called nitre-beds.

Nitrate of potash is an anhydrous salt,—it contains simply nitric acid and potash, without any water of crystallization; still, in many cases, a little water is retained mechanically between the interstices of the crystals, and therefore it is better to dry before use. This may be done by laying it in a state of fine powder upon blotting-paper, close to a fire, or upon a heated metallic plate.

Nitrate of Baryta.

Symbol, BaO NO{5}. Atomic weight, 131.

Nitrate of baryta forms octahedral crystals, which are anhydrous. It is considerably less soluble than the chloride of barium, requiring 12 parts of cold and 4 of boiling water for solution. It may be substituted for the nitrate of lead in the preparation of protonitrate of iron.

Nitrate of Lead.

Symbol, PbO NO{5}. Atomic weight, 166.

Nitrate of lead is obtained by dissolving the metal, or the oxide of lead, in excess of nitric acid, diluted with 2 parts of water. It crystallizes on evaporation in white anhydrous tetrahedra and octahedra, which are hard, and decrepitate on being heated; they are soluble in 8 parts of water at 60°.

Nitrate of lead forms with sulphuric acid, or soluble sulphates, a white precipitate, which is the insoluble sulphate of lead. The Iodide of lead is also very sparingly soluble in water.

Nitrate of Silver. (See Silver, Nitrate of.)

Nitro-Hydrochloric Acid.

Symbol, NO{4} + Cl.

This liquid is the aqua-regia of the old alchemists. It is produced by mixing nitric and hydrochloric acids: the oxygen contained in the former combines with the hydrogen of the latter, forming water and liberating chlorine, thus:—

NO{5} + HCl = NO{4} + HO + Cl.

The presence of free chlorine confers on the mixture the power of dissolving gold and platinum, which neither of the two acids possesses separately. In preparing aqua-regia it is usual to mix one part, by measure, of nitric acid with four of hydrochloric acid, and to dilute with an equal bulk of water. The application of a gentle heat assists the solution of the metal; but if the temperature rises to the boiling point, a violent effervescence and escape of chlorine takes place.

Oxygen.

Symbol, O. Atomic weight, 8.

Oxygen gas may be obtained by heating nitrate of potash to redness, but in this case it is contaminated with a portion of nitrogen. The salt termed chlorate of potash (the composition of which is closely analogous to that of the nitrate, chlorine being substituted for nitrogen) yields abundance of pure oxygen gas on the application of heat, leaving behind chloride of potassium.

Chemical Properties.—Oxygen combines eagerly with many of the chemical elements, forming oxides. This chemical affinity however is not well seen when the elementary body is exposed to the action of oxygen in the gaseous form. It is the nascent oxygen which acts most powerfully as an oxidizer. By nascent oxygen is meant oxygen on the point of separation from other elementary atoms with which it was previously associated; it may then be considered to be in the liquid form, and hence it comes more perfectly into contact with the particles of the body to be oxidized.

Illustrations of the superior chemical energy of nascent oxygen are numerous, but none perhaps are more striking than the mild and gradual oxidizing influence exerted by atmospheric air, as compared with the violent action of nitric acid and bodies of that class which contain oxygen loosely combined.

Oxymel.

This syrup of honey and vinegar is prepared as follows:—Take of

Stand the pot containing the honey in boiling water until a scum rises to the surface, which is to be removed two or three times. Then add the acetic acid and water, and skim once more if required. Allow to cool, and it will be fit for use.

Potash.

Symbol, KO + HO. Atomic weight, 57.

Potash is obtained by separating the carbonic acid from carbonate of potash by means of caustic lime. Lime is a more feeble base than potash, but the carbonate of lime, being insoluble in water, is at once formed on adding milk of lime to a solution of carbonate of potash.

Properties.—Usually met with in the form of solid lumps, or in cylindrical sticks, which are formed by melting the potash and running it into a mould. It always contain some atoms of water, which cannot be driven off by the application of heat.

Potash is soluble almost to any extent in water, much heat being evolved. The solution is powerfully alkaline and acts rapidly upon the skin; it dissolves fatty and resinous bodies, converting them into soaps; Solution of potash absorbs carbonic acid quickly from the air, and should therefore be preserved in stoppered bottles; the glass stoppers must be wiped occasionally, in order to prevent them from becoming immovably fixed by the solvent action of the potash upon the silica of the glass.

The liquor potassÆ of the London Pharmacopoeia has a sp. gr. of 1·063, and contains about 5 per cent; of real potash. It is usually contaminated with carbonate of potash, which causes it to effervesce on the addition of acids; also, to a less extent, with sulphate of potash, chloride of potassium, silica, etc.

Potash, Carbonate of.

Symbol, KO CO{2}. Atomic weight, 70.

The impure carbonate of potash, termed pearlash, is obtained from the ashes of wood and vegetable matter, in the same manner as carbonate of soda is prepared from the ashes of seaweeds. Salts of potash and of soda appear essential to vegetation, and are absorbed and approximated by the living tissues of the plant. They exist in the vegetable structure combined with organic acids in the form of salts, like the oxalate, tartrate, etc., which when burned are converted into carbonates.

Properties.—The pearlash of commerce contains large and variable quantities of chloride of potassium, sulphate of potash, etc. A purer carbonate is sold, which is free from sulphates, and with only a trace of chlorides. Carbonate of potash is a strongly alkaline salt, deliquescent, and soluble in twice its weight of cold water; insoluble in alcohol, and employed to deprive it of water.

Pyrogallic Acid.

Symbol, C{8}H{4}O{4} (Stenhouse). Atomic weight. 84.

The term pyro prefixed to gallic acid implies that the new substance is obtained by the action of heat upon that body. At a temperature of about 410° Fahr., gallic acid is decomposed, and a white sublimate forms, which condenses in lamellar Crystals; this is pyrogallic acid.

Pyrogallic acid is very soluble in cold water, and in alcohol and ether; the solution decomposes and becomes brown by exposure to the air. It gives an indigo blue color with protosulphate of iron, which changes to dark green if any persulphate be present.

Although termed an acid, this substance is strictly neutral; it does not redden litmus-paper, and forms no salts. The addition of potash or soda decomposes pyrogallic acid, at the same time increasing the attraction for oxygen; hence this mixture may conveniently be employed for absorbing the oxygen contained in atmospheric air. The compounds of silver and gold are reduced by pyrogallic acid even more rapidly than by gallic acid, the reducing agent absorbing the oxygen, and becoming converted into carbonic acid and a brown matter insoluble in water.

Commercial pyrogallic acid is often contaminated with empyreumatic oil, and also with a black insoluble substance known as metagallic acid, which is formed when the heat is raised above the proper temperature in the process of manufacture.

Sel D'or. (See Gold, Hyposulphite of.)

Silver.

Symbol, Ag. Atomic Weight, 108.

This metal, the luna or diana of the alchemists, is found native in Peru and Mexico; it occurs also in the form of sulphuret of silver.

When pure it has a sp. gr. of 10·5, and is very malleable and ductile; melts at a bright red heat. Silver does not oxidize in the air, but when exposed to an impure atmosphere containing traces of sulphuretted hydrogen, it is slowly tarnished from formation of sulphuret of silver. It dissolves in sulphuric acid, but the best solvent is nitric acid.

The standard coin of the realm is an alloy of silver and copper, containing about one-eleventh of the latter metal. It may be converted into nitrate of silver, sufficiently pure for photographic purposes, by dissolving it in nitric acid and evaporating the solution to the crystallizing point: or, if the quantity be small, the solution may be boiled down to complete dryness, and the residue fused strongly; which decomposes the nitrate of copper, but leaves the greater portion of the silver salt unaffected. (N. B. Nitrate of silver which has undergone fusion contains nitrite of silver, and will require the addition of acetic acid if used for preparing the collodion sensitive film.)

Silver, Ammonio-Nitrate of.

Crystallized nitrate of silver absorbs ammoniacal gas rapidly, with production of heat sufficient to fuse the resulting compound, which is white, and consists of 100 parts of the nitrate + 29·5 of ammonia. The compound however which photographers employ under the name of ammonio-nitrate of silver, may be viewed more simply as a solution of the oxide of silver in ammonia, without reference to the nitrate of ammonia necessarily produced in the reaction.

Very strong ammonia, in acting upon oxide of silver, converts it into a black powder, termed fulminating silver, which possesses the most dangerous explosive properties. Its composition is uncertain. In preparing ammonio-nitrate of silver by the common process, the oxide first precipitated occasionally leaves a little black powder behind, on re-solution; this does not appear, however, according to the observations of the author, to be fulminating silver.

In sensitizing salted paper by the ammonio-nitrate of silver, free ammonia is necessarily formed. Thus:—

Chloride of ammonium + oxide of silver in ammonia = chloride of silver + ammonia + water.

Silver, Oxide of.

Symbol, AgO. Atomic weight, 116.

If a little potash or ammonia be added to solution of nitrate of silver, a brown substance is formed, which, on standing, collects at the bottom of the vessel. This is oxide of silver, displaced from its previous state of combination with nitric acid by the stronger oxide, potash. Oxide of silver is soluble to a very minute extent in pure water, the solution possessing an alkaline reaction to litmus; it is easily dissolved by nitric or acetic acid, forming a neutral nitrate or acetate; also soluble in ammonia (ammonio-nitrate of silver), and in nitrate of ammonia hyposulphite of soda, and cyanide of potassium. Long exposure to light converts it into a black substance, which is probably a suboxide.

Properties of the Suboxide of Silver.—Suboxide of silver bears the same relation to the ordinary brown protoxide of silver that subchloride bears to protochloride of silver.

It is a black powder, which assumes the metallic lustre on rubbing, and when treated with dilute acids is resolved into protoxide of silver which dissolves, and metallic silver.

Silver, Chloride of.

Symbol, AgCl. Atomic weight, 144.

Preparation of Chloride of Silver by double decomposition.—In order to illustrate this, take a solution in water of chloride of sodium or "common salt," and mix it with a solution containing nitrate of silver; immediately a dense, curdy, white precipitate falls, which is the substance in question.

In this reaction the elements change places; the chlorine leaves the sodium with which it was previously combined, and crosses over to the silver; the oxygen and nitric acid are released from the silver, and unite with the sodium: thus

Chloride of sodium + nitrate of silver = Chloride of silver + nitrate of soda.

This interchange of elements is termed by chemists double decomposition.

The essential requirements in two salts intended for the preparation of chloride of silver, are simply that the first should contain chlorine, the second silver, and that both should be soluble in water; hence the chloride of potassium or ammonium may be substituted for the chloride of sodium, and the sulphate or acetate for the nitrate of silver.

In preparing chloride of silver by double decomposition, the white clotty masses which first form must be washed repeatedly with water, in order to free them from soluble nitrate of soda, the other product of the change. When this is done, the salt is in a pure state, and may be dried, etc., in the usual way.

Properties of Chloride of Silver.—Chloride of silver differs in appearance from the nitrate of silver. It is not met with in crystals, but forms a soft white powder resembling common chalk or whiting. It is tasteless and insoluble in water; unaffected by boiling with the strongest nitric acid, but sparingly dissolved by concentrated hydrochloric acid.

Ammonia dissolves chloride of silver freely, as do solutions of hyposulphite of soda and cyanide of potassium. Concentrated solutions of alkaline chlorides, iodides, and bromides are likewise solvents of chloride of silver, but to a limited extent.

Dry chloride of silver heated to redness fuses, and concretes on cooling into a tough and semi-transparent substance, which has been termed horn silver or luna cornea.

Placed in contact with metallic zinc or iron acidified with dilute sulphuric acid, chloride of silver is reduced to the metallic state, the chlorine passing to the other metal under the decomposing influence of the galvanic current which is established.

Preparation and properties of the Subchloride of Silver.—If a plate of polished silver be dipped in solution of perchloride of iron, or of bichloride of mercury, a black stain is produced, the iron or mercury salt losing a portion of chlorine, which passes to the silver and converts it superficially into subchloride of silver. This compound differs from the white chloride of silver in containing less chlorine and more of the metallic element; the composition of the latter being represented by the formula AgCl, that of the former may perhaps be written as Ag{2}Cl. (?)

Subchloride of silver is interesting to the photographer as corresponding in properties and composition with the ordinary chloride of silver blackened by light. It is a pulverulent substance of a bluish-black color, which is decomposed by ammonia, hyposulphite of soda, and cyanide of potassium, into chloride of silver which dissolves, and insoluble metallic silver.

Silver, Bromide of.

Symbol, AgBr. Atomic weight, 186.

This substance so closely resembles the corresponding salts containing, chlorine and iodine, that a short notice of it will suffice.

Bromide of silver is prepared by exposing a silvered plate to the vapor of bromine, or by adding solution of bromide of potassium to nitrate of silver. It is an insoluble substance, slightly yellow in color, and distinguished from iodide of silver by dissolving in strong ammonia and in chloride of ammonium. It is freely soluble in hyposulphite of soda and in cyanide of potassium.

Silver, Citrate of. (See Citric Acid.)

Silver, Iodide of.

Symbol, AgI. Atomic weight, 234.

Preparation and Properties of Iodide of Silver.—Iodide of silver may be formed in an analogous manner to the chloride, viz. by the direct action of the vapor of iodine upon metallic silver, or by double decomposition between solutions of iodide of potassium and nitrate of silver.

When prepared by the latter mode it forms an impalpable powder, the color of which varies slightly with the manner of precipitation. If the iodide of potassium be in excess, the iodide of silver falls to the bottom of the vessel nearly white; but with an excess of nitrate of silver it is of a straw-yellow tint. This point may be noticed, because the yellow salt is the one adapted for photographic use, the other being insensible to the influence of light.

Iodide of silver is tasteless and inodorous; insoluble in water and in dilute nitric acid. It is scarcely dissolved by ammonia, which serves to distinguish it from the chloride of silver, freely soluble in that liquid. Hyposulphite of soda and cyanide of potassium both dissolve iodide of silver; it is also soluble in solutions of the alkaline bromides and iodides.

Silver, Fluoride of.

Symbol, AgF. Atomic weight, 127.

This compound differs from those just described in being soluble in water. The dry salt fuses on being heated, and is reduced by a higher temperature, or by exposure to light.

Silver, Sulphuret of.

Symbol, AgS. Atomic weight, 124.

This compound is formed by the action of sulphur upon metallic silver, or of sulphuretted hydrogen, or hydrosulphate of ammonia, upon the silver salts; the decomposition of hyposulphite of silver also furnishes the black sulphuret.

Sulphuret of silver is insoluble in water, and nearly so in those substances which dissolve the chloride, bromide, and iodide, such as ammonia, hyposulphites, cyanides, etc.; but it dissolves in nitric acid, being converted into soluble sulphate and nitrate of silver.

Silver, Nitrate of.

Symbol, AgO NO{5}. Atomic weight, 170.

Nitrate of silver is prepared by dissolving metallic silver in nitric acid. Nitric acid is a powerfully acid and corrosive substance, containing two elementary bodies united in definite proportions. These are nitrogen and oxygen; the latter being present in greatest quantity.

Nitric acid is a powerful solvent for the metallic bodies generally. To illustrate its action in that particular, as contrasted with other acids, place pieces of silver foil in two test-tubes, the one containing dilute sulphuric, the other dilute nitric acid; on the application of heat a violent action soon commences in the latter, but the former is unaffected. In order to understand the cause of the difference, it must be borne in mind that when a metallic substance dissolves in an acid, the nature of the solution is unlike that of an aqueous solution of salt or sugar. If you take salt water, and boil it down until the whole of the water has evaporated, you obtain the salt again, with properties the same as at first; but if a similar experiment be made with a solution of silver in nitric acid, the result is different: in that case you do not get metallic silver on evaporation, but silver combined with oxygen and nitric acid, both of which are tightly retained, being, in fact, in a state of chemical combination with the metal.

If we closely examine the effects produced by treating silver with nitric acid, we find them to be of the following nature:—first, a certain amount of oxygen is imparted to the metal, so as to form an oxide, and afterwards this oxide dissolves in another portion of the nitric acid, producing nitrate of the oxide, or, as it is shortly termed, nitrate of silver.

It is therefore the instability of nitric acid, its proneness to part with oxygen, which renders it superior to sulphuric acid in the experiment of dissolving silver. Nitric acid stands high in the list of "oxidizing agents," and it is important that the photographer should bear this fact in mind.

Properties of Nitrate of Silver.—In the preparation of nitrate of silver, when the metal has dissolved, the solution is boiled down in order to drive off the excess of nitric acid, and set aside to crystallize. The salt, however, as so obtained is still acid to test-paper, and requires either recrystallization, or a careful heating to about 300° Fahrenheit, to render it perfectly neutral.

Pure nitrate of silver occurs in the form of white crystalline plates, which are very heavy and dissolve readily in an equal weight of cold water. The solubility is much lessened by the presence of free nitric acid, and in the concentrated nitric acid the crystals are almost insoluble. Boiling alcohol takes up about one-fourth part of its weight of the crystallized nitrate, but deposits nearly the whole on cooling. Nitrate of silver has an intensely bitter and nauseous taste; acting as a caustic, and corroding the skin by a prolonged application. Its aqueous solution is perfectly neutral to test-paper.

Heated in a crucible the salt melts, and when poured into a mould and solidified, forms the lunar caustic of commerce. At a still higher temperature it is decomposed, and bubbles of oxygen gas are evolved. The melted mass, cooled and dissolved in water, leaves behind a black powder, and yields a solution which is faintly alkaline to test-paper. The alkalinity depends upon the presence of nitrite of silver associated with excess of oxide, in the form probably of a basic or sub-nitrite of silver.[B]

Solution of nitrate of silver is decomposed by iron, zinc, copper, mercury, etc., the nitric acid and oxygen passing to the other metal, and metallic silver being precipitated.

Silver, Nitrite of.

Symbol, AgO NO{3}. Atomic weight, 154.

Nitrite of silver is a compound of nitrous acid, or NO{3}, with oxide of silver. It is formed by heating nitrate of silver, so as to drive off a portion of its oxygen, or more conveniently, by mixing nitrate of silver and nitrate of potash in equal parts, fusing strongly, and dissolving in a small quantity of boiling water; on cooling, the nitrite crystallizes out, and may be purified by pressing in blotting paper. Mr. Hadow describes an economical method of preparing nitrite of silver in quantity, viz. by heating 1 part of starch in 8 of nitric acid of 1·25 specific gravity, and conducting the evolved gases into a solution of pure carbonate of soda until effervescence has ceased. The nitrite of soda thus formed is afterwards added to nitrate of silver in the usual way.

Properties.—Nitrite of silver is soluble in 120 parts of cold water; easily soluble in boiling water, and crystallizes, on cooling, in long slender needles. It has a certain degree of affinity for oxygen, and tends to pass into the condition of nitrate of silver; but it is probable that its photographic properties depend more upon a decomposition of the salt and liberation of nitrous acid.

Properties of Nitrous Acid.—This substance possesses very feeble acid properties, its salts being decomposed even by acetic acid. It is an unstable body, and splits up, in contact with water, into binoxide of nitrogen and nitric acid. The peroxide of nitrogen, NO{4}, is also decomposed by water and yields the same products.

Silver, Acetate of.

Symbol, AgO (C{4}H{3}O{3}). Atomic weight, 167.

This is a difficultly soluble salt, deposited in lamellar crystals when an acetate is added to a strong solution of nitrate of silver. If acetic acid be used in place of an acetate, the acetate of silver does not fall so readily, since the nitric acid which would then be liberated impedes the decomposition.

Silver, Hyposulphite of.

Symbol, AgO S{2}O{3} . Atomic weight, 164.

In order to understand, more fully how decomposition of hyposulphite of silver may affect the process of fixing, the peculiar properties of this salt should be studied. With this view nitrate of silver and hyposulphite of soda may be mixed in equivalent proportions, viz. about twenty-one grains of the former salt to sixteen grains of the latter, first dissolving each in separate vessels in half an ounce of distilled water. These solutions are to be added to each other and well agitated; immediately a dense deposit forms, which is hyposulphite of silver.

At this point a curious series of changes commences. The precipitate, at first white and curdy, soon alters in color: it becomes canary-yellow, then of a rich orange-yellow, afterwards liver-color, and finally black. The rationale of these changes is explained to a certain extent by studying the composition of the hyposulphite of silver.

The formula for this substance is as follows:—

AgO S{2}O{2},

But AgO S{2}O{2} plainly equals AgS, or sulphuret of silver, and SO{3}, or sulphuric acid. The acid reaction assumed by the supernatant liquid is due therefore to sulphuric acid, and the black substance formed is sulphuret of silver. The yellow and orange-yellow compounds are earlier stages of the decomposition, but their exact nature is uncertain.

The instability of hyposulphite of silver is principally seen when, it is in an isolated state: the presence of an excess of hyposulphite of soda renders it more permanent, by forming a double salt.

In fixing photographic prints this brown deposit of sulphuret of silver is very liable to form in the bath and upon the picture; particularly so when the temperature is high. To obviate it observe the following directions:—It is especially in the reaction between nitrate of silver and hyposulphite of soda that the blackening is seen; the chloride and other insoluble salts of silver being dissolved, even to saturation, without any decomposition of the hyposulphite first formed. Hence, if the print be washed in water to remove the soluble nitrate, a very much weaker fixing bath than usual may be employed. This plan, however, involving a little additional trouble, is, on that account, often objected to, and, when such is the case, a concentrated solution of hyposulphite of soda must be used, in order to dissolve off the white hyposulphite of silver before it begins to decompose. When the proofs are taken at once from the printing frame and immersed in a dilute bath of hyposulphite (one part of the salt to six or eight of water), a shade of brown may often be observed to pass over the surface of the print, and a large deposit of sulphuret of silver soon forms as the result of this decomposition. On the other hand, with a strong hyposulphite bath there is little or no discoloration and the black deposit is absent.

But even if, by a preliminary removal of the nitrate of silver, the danger of blackening be in a great measure obviated, yet the print must not be taken out of the fixing bath too speedily, or some appearance of brown patches, visible by transmitted light, may occur.

Each atom of nitrate of silver requires three atoms of hyposulphite of soda to form the sweet and soluble double salt, and hence, if the action be not continued sufficiently long, another compound will be formed almost tasteless and insoluble. Even immersion in a new bath of hyposulphite of soda does not fix the print when once the yellow stage of decomposition has been established. This yellow salt is insoluble in hyposulphite of soda, and consequently remains in the paper.

Sugar of Milk. (See Milk.)

Sulphuretted Hydrogen. (See Hydrosulphuric Acid.)

Sulphuric Acid.

Symbol, SO{3}. Atomic weight, 40.

Sulphuric acid maybe formed by oxidizing sulphur with boiling nitric acid; but this plan would be too expensive to be adopted on a large scale. The commercial process for the manufacture of sulphuric acid is exceedingly ingenious and beautiful, but it involves reactions which are too complicated to admit of a superficial explanation. The sulphur is first burnt into gaseous sulphurous acid (SO{2}), and then, by the agency of binoxide of nitrogen gas, an additional atom of oxygen is imparted from the atmosphere, so as to convert the SO{2} into SO{3}, or sulphuric acid.

Properties.—Anhydrous sulphuric acid is a white crystalline solid. The strongest liquid acid always contains one atom of water, which is closely associated with it, and cannot be driven off by the application of heat.

This mono-hydrated sulphuric acid, represented by the formula HO SO{3}, is a dense fluid, having a specific gravity of about 1·845; boils at 620°, and distils without decomposition. It is not volatile at common temperatures, and therefore does not fume in the same manner as nitric or hydrochloric acid. The concentrated acid may be cooled down even to zero without solidifying; but a weaker compound, containing twice the quantity of water, and termed glacial sulphuric acid, crystallizes at 40° Fahr. Sulphuric acid is intensely acid and caustic, but it does not destroy the skin or dissolve metals so readily as nitric acid. It has an energetic attraction for water, and when the two are mixed, condensation ensues, and much heat is evolved; four parts of acid and one of water produce a temperature equal to that of boiling water. Mixed with aqueous nitric acid, it forms the compound known as nitro-sulphuric acid.

Sulphuric acid possesses intense chemical powers, and displaces the greater number of ordinary acids from their salts. It chars organic substances, by removing the elements of water, and converts alcohol into ether in a similar manner. The strength of a given sample of sulphuric acid may generally be calculated from its specific gravity, and a table is given by Dr. Ure for that purpose.

Impurities of Commercial Sulphuric Acid.—The liquid acid sold as oil of vitriol is tolerably constant in composition, and seems to be as well adapted for photographic use as the pure sulphuric acid, which is far more expensive. The specific gravity should be about 1·836 at 60°. If a drop, evaporated upon platinum foil, gives a fixed residue, probably bisulphate of potash is present. A milkiness, on dilution, indicates sulphate of lead.

Test for Sulphuric Acid.—If the presence of sulphuric acid, or a soluble sulphate, be suspected in any liquid, it is tested for by adding a few drops of dilute solution of chloride of barium, or nitrate of baryta. A white precipitate, insoluble in nitric acid, indicates sulphuric acid. If the liquid to be tested is very acid, from nitric or hydrochloric acid, it must be largely diluted before testing, or a crystalline precipitate will form, caused by the sparing solubility of the chloride of barium itself in acid solutions.

Sulphurous Acid.

Symbol, SO{2}. Atomic weight, 32.

This is a gaseous compound, formed by burning sulphur in atmospheric air or oxygen gas; also by heating oil of vitriol in contact with metallic copper, or with charcoal.

When an acid of any kind is added to hyposulphite of soda, sulphurous acid is formed as a product of the decomposition of hyposulphurous acid, but it afterwards disappears from the liquid by a secondary reaction, resulting in the production of trithionate and tetrathionate of soda.

Properties.—Sulphurous acid possesses a peculiar and suffocating odor, familiar to all in the fumes of burning sulphur. It is a feeble acid, and escapes with effervescence, like carbonic acid, when its salts are treated with oil of vitriol. It is soluble in water.

Water.

Symbol, H{2}O. Atomic weight, 9.

Water is an oxide of hydrogen, containing single atoms of each of the gases.

Distilled water is water which has been vaporized and again condensed: by this means it is freed from earthy and saline impurities, which, not being volatile, are left in the body of the retort. Pure distilled water leaves no residue on evaporation, and should remain perfectly clear on the addition of nitrate of silver, even when exposed to the light; it should also be neutral to test-paper.

The condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter, which discolors nitrate of silver, and is therefore injurious.

Rain-water, having undergone a natural process of distillation, is free from inorganic salts, but it usually contains a minute portion of ammonia, which gives it an alkaline reaction to test-paper. It is very good for photographic purposes if collected in clean vessels, but when taken from a common rain-water tank should always be examined, and if much organic matter be present, tinging it of a brown color and imparting an unpleasant smell, it must be rejected.

Spring or river water, commonly known as "hard water," usually contains sulphate of lime, and carbonate of lime dissolved in carbonic acid: also chloride of sodium in greater or less quantity. On boiling the water, the carbonic acid gas is evolved, and the greater part of the carbonate of lime (if any is present) deposits, forming an earthy incrustation on the boiler.

In testing water for sulphates and chlorides, acidify a portion with a few drops of pure nitric acid, free from chlorine (if this is not at hand, use pure acetic acid); then divide it into two parts, and add to the first a dilute solution of chloride of barium, and to the second nitrate of silver,—a milkiness indicates the presence of sulphates in the first case or of chlorides in the second. The photographic nitrate bath cannot be used as a test, since the iodide of silver it contains is precipitated on dilution, giving a milkiness which might be mistaken for chloride of silver.

Common hard water can often be used for making a nitrate bath when nothing better is at hand. The chlorides it contains are precipitated by the nitrate of silver, leaving soluble nitrates in solution, which are not injurious. The carbonate of lime, if any is present, neutralizes free nitric acid, rendering the bath alkaline in the same manner as carbonate of soda. Sulphate of lime, usually present in well water, is said to exercise a retarding action upon the sensitive silver salts, but on this point the writer is unable to give certain information.

Hard water is not often sufficiently pure for the developing fluids. The chloride of sodium it contains decomposes the nitrate of silver upon the film, and the image cannot be brought out perfectly. The New River water, however supplied to many parts of London, is almost free from chlorides and answers very well. In other cases a few drops of nitrate of silver solution may be added to separate the chlorine, taking care not to use a large excess.

Black Varnish.

Asphaltum, dissolved in Spirits or Oil of Turpentine.—The asphaltum may be coarsely pulverized and put into a bottle containing the turpentine, and in a few hours, if it be occasionally shaken, it will be dissolved and ready for use. It should be of about the consistency of thick paste.

I use the above, but will now give two more compositions, for any who may wish to adopt them:

Black Japan.—Boil together a gallon of boiled linseed oil, 8 ounces of amber, and 3 ounces of asphaltum. When sufficiently cool, thin it with oil of turpentine.

Brunswick Black.—Melt 4 lbs. of asphaltum, add 2 lbs. of hot boiled linseed oil, and when sufficiently cool, add a gallon of oil of turpentine.

The following is from Humphrey's Journal, Vol. viii, number 16.

Black Varnish.—I generally purchase this from the dealer; but I have made an article which answered the purpose well, by dissolving pulverized asphaltum in spirits of turpentine. Any of the black varnishes can be improved by the addition of a little bees'-wax to it. It is less liable to crack and gives an improved gloss.

Before closing this chapter, it has been thought advisable to remark, that one of the most important departments of Photography is the practice of its chemistry. Many of the annoying failures experienced by those who are just engaging in the practice of the art, arise from the want of good and pure chemical agents, and the most certain way to avoid this, is to purchase them only from persons who thoroughly understand both their nature and mode of application. As many who may read this work might wish to know the prices of the various articles employed in the practice of the processes given, they can be informed by addressing the author, who will furnish them with a printed Price List.

PRACTICAL DETAILS

OF THE

POSITIVE

OR

AMBROTYPE PROCESS.