The subject of the purity of potable waters possesses the highest degree of importance in its sanitary relations, and, particular attention has been bestowed upon methods of analysis that would serve to indicate the character and significance of existing impurities. The earlier processes of examination, which chiefly consisted in the determination of the mineral constituents of water, while of use in furnishing an idea of the general nature of the water regarded as an inorganic solution, almost totally failed to reveal the presence of the more subtle and important organic contaminations which are now known to exert an active influence in the propagation of zymotic diseases. During the past few years, decided progress has been attained in the analytical methods employed. Little is known of the exact nature of the organic constituents present in water that has received sewage contamination. They may be either of vegetable or animal origin, and it appears to be very probable that they constitute organised germs. But, although we are still unable to determine the constitution of these deleterious ingredients, it is at present possible to approximately ascertain the hygienic character of drinking water, and to distinguish, with a fair degree of accuracy, between a good and a bad sample. In arriving at a conclusion regarding the sanitary quality of water, it is, however, also needful to take into consideration the origin and surrounding conditions which affect the chances of contamination. Most of the more recent methods of water analysis are based upon the fact, that the putrefactive decomposition of harmful organic matter is attended by the genesis of certain compounds (such as ammonia, nitrites, and nitrates), of which quantitative estimations can be made. For the purpose of ascertaining the character of a potable water, the following determinations are usually necessary:—

1. Colour, odour, and taste.

2. Total solid matter and loss on ignition.

3. Organic matter in solution.

4. Chlorine.

5. Ammonia, free and albuminoid.

6. Nitrogen, as nitrites and nitrates.

Certain precautions should be observed in the collection of samples of water intended for examination. It is indispensable for this purpose to employ scrupulously clean glass stoppered bottles, which are washed out several times with the water previous to being filled. If a well or stream is to be sampled, the bottle should be entirely immersed in the water some distance from the sides of the stream, and, if taken from a pump or pipe, the latter should be cleansed by first running a considerable quantity of the water before charging the bottle.

1. Colour, odour, and taste.—The colour is best determined by filling a glass cylinder, about 2 feet in height, with the sample, placing it upon a white surface and observing the tint produced; or, by the use of a coloured glass tube of the same length, which is provided with glass plates attached at each end, and is filled with the sample and viewed when held towards a sheet of white paper.

As a rule, pure water exhibits a light-bluish tint, a yellowish hue being generally considered a suspicious indication; but it frequently occurs that a perfectly colourless water is bad, and one possessing a decided colour may prove to be at least, fair in quality. The odour of the sample is ascertained by placing a corked bottle, one-half filled with the water, in a warm place (at about 38°) for some time, and then shaking the bottle, withdrawing the stopper and immediately testing the odour. Pure water should be free from much perceptible odour of any kind, and more especially from one of a disagreeable nature. The same remark applies to the taste. Water should be practically tasteless, even when warmed. It frequently happens, however, that a water may be highly contaminated with deleterious organic impurities, and still remain devoid of any marked unpleasant taste. There are few simple tests of any value which will reveal at once the sanitary quality of drinking water. One, sometimes employed, is to fill a clean quart bottle about three-fourths full with the suspected sample, and dissolve in it a teaspoonful of fine granulated white sugar. The bottle is then corked and allowed to remain in a warm place for two days, when, in the presence of sewage contamination, it will become cloudy or milky.[119] According to Wanklyn and Chapman,[120] if a brownish colour or precipitate is produced upon the addition of 1·5 c.c. of Nessler’s reagent (see p. 208) to 100 c.c. of the water, it should be considered unfit for domestic use.

2. Total solid residue and loss on ignition.—500 c.c. of the water under examination are introduced, in small portions at a time, into a tared platinum dish, and evaporated to dryness over the water-bath, the residue being subsequently dried for three or four hours in an air-bath at 100°. The solid residue obtained, multiplied by 200, represents parts in 100,000: or, by 140, grains per imperial gallon. It is usually considered that, unless the proportion of total solids exceeds 40 grains per Imperial gallon (32 grains per U.S. gallon, or about 56·5 parts per 100,000), the water need not be objected to for drinking purposes on this ground alone. The volatile and organic matters are determined by igniting the solid residue, which is afterwards allowed to cool. It is then moistened with a little carbonic acid water or solution of ammonium carbonate, dried to constancy at 130°, and the organic matter estimated by the decrease in weight. Formerly, this process was chiefly depended upon for determining the proportion of organic substances contained in water. It is open to numerous serious objections, among which are, that it may afford a result either below or above that correctly representing the quantity of organic ingredients present in the sample. The first case takes place when a portion of the organic matter is decomposed during the process of evaporation, and is quite liable to occur; the second case takes place when the water contains nitrates, which would be decomposed upon ignition. The method, however, possesses some value, and is still often resorted to as affording a general idea of the proportion of organic contamination present, the degree of blackening of the solid residue during the process of ignition being, at least, a useful qualitative indication.

3. Organic matter in solution.—A method frequently employed for this determination is based upon the supposition that the amount of potassium permanganate required to oxidise the organic constituents contained in water would serve as a criterion of its sanitary value. It is generally known as the “Forchammer” or “oxygen” process, and, although of undoubted service in comparing the quality of samples of very impure water, it is defective in the following important respects: Different organic substances are not affected to an equal extent by potassium permanganate; albumen, for instance, being far less easily oxidised than other compounds, and the value of the results afforded is vitiated by the presence of certain inorganic bodies, such as nitrites, sulphuretted hydrogen, ferrous salts, etc. It has been stated, that the more deleterious and putrescent organic ingredients of water are those most readily affected by the permanganate solution. As modified and improved by Miller[121] and by Tidy,[122] the process consists substantially in adding an excess of a standard solution of potassium permanganate to a measured quantity of the water under examination (acidulated with sulphuric acid), and then determining the excess of permanganate used by means of sodium hyposulphite and potassium iodide. The following solutions are required:—

Potassium Permanganate.—0·395 gramme of the salt is dissolved in 1 litre of distilled water; 10 c.c. of this solution represent 0·001 gramme of available oxygen.

Sodium Hyposulphite.—One gramme of the salt is dissolved in a litre of water.

Starch solution.—One gramme of starch is triturated with about 20 c.c. of boiling water, and the mixture allowed to stand at rest over night, after which the clear supernatant solution is drawn off.

Pure distilled Water.—This is prepared by digesting 10 litres of distilled water with 10 grammes of potassium hydroxide and 2 grammes of potassium permanganate in a still provided with an inverted condenser at 100° for twenty-four hours, after which the water is distilled, separate portions being frequently tested with Nessler’s solution; the distillate is not reserved for use until this reagent ceases to produce a brownish coloration.

The determination proper is executed as follows:—Two flasks are first thoroughly cleansed by washing with concentrated sulphuric acid, and subsequently with water; 250 c.c. of the water to be examined are introduced into one, and the same volume of the pure distilled water, prepared as above, is placed in the other. 10 c.c. of dilute sulphuric acid (1 part pure acid and 8 parts distilled water) and 10 c.c. of the potassium permanganate solution are now added to each flask, both then being put aside for three hours. Two drops of a 10 per cent. solution of potassium iodide are next added to the flasks, and the amount of iodine liberated (which is equivalent to the quantity of permanganate unacted upon by the water) is determined by titration with the sodium hyposulphite solution. The precise end of the reaction is ascertained by means of a few drops of the starch paste, the hyposulphite being added to each flask until the blue colour produced by the starch disappears. The quantities of solution used in each titration are then read off.

The amount of permanganate consumed is equal to A-B, where A represents the hyposulphite used with the distilled water, and B, that used with the sample under examination, and the proportion of oxygen which is consumed by the water tested, can be calculated by the formula:—

(A - B) a A

in which a is the available oxygen in the added permanganate. For example, if 10 c.c. of permanganate (= 0·001 gramme available oxygen) are added to the 250 c.c. (= ¼ litre) contained in each flask, and the distilled water required 35 c.c., the sample 15 c.c., of the hyposulphite solution, the proportion of oxygen consumed by the ¼ litre of water, would be

(35 - 15) × ·001 35

= ·000571, which represents ·228 parts of oxygen in 100,000 parts of water.

In applying the preceding test, it is requisite that the flasks should be kept at a particular temperature, such as 27°. The presence of putrescent and readily oxidised organic matter or nitrites, which indicates dangerous contamination, is recognised by the absorption of any considerable proportion of oxygen in the space of two minutes. According to Dr. Tidy, 100,000 parts of water of various degrees of purity, absorb the following amount of oxygen in three hours:—

4. Chlorine.—The importance attached to the estimation of chlorine in potable waters is derived from the fact that this element enters largely into the food of men and animals, and is thrown off in their excreta. This, naturally, contributes to the sewage contamination to which water is often exposed. Water, however, may take up a certain proportion of chlorides from the geological strata through which it passes, and it is of importance to bear this fact in mind in forming a conclusion as to the significance of the results afforded by this determination. It is, likewise, to be remembered that vegetable organic pollution would escape detection were the quantity of chlorine contained alone taken into consideration. The determination is conveniently made as follows:—50 c.c. of the water are introduced into a beaker, a drop or two of a concentrated and neutral solution of potassium chromate added, and then a standard solution of silver nitrate very cautiously added from a burette, drop by drop, until a faint but permanent red tint is produced. If the silver solution is prepared by dissolving 2·394 grammes of the nitrate in 1 litre of distilled water, the number of c.c. required to cause the reddish coloration directly indicates the parts of chlorine present in 100,000 parts of the water examined. According to Frankland, 100,000 parts of water from various sources contain the following proportions of chlorine:—

Watts’ ‘Dictionary of Chemistry’ quotes the proportions below:—

The amount of chlorine contained in sewage is stated to range from 6·5 to 21·5 parts, the average being 11·54 parts.[123] It is generally considered that a proportion in excess of 5 parts in 100,000 parts of a drinking water, which is not liable to be affected by mineral admixture, is to be ascribed to organic contamination.

5. Ammonia, free and albuminoid.—It has already been mentioned that the decomposition of the nitrogenous organic impurities present in polluted water results in the production, first, of ammonia, then of nitrites and nitrates, and, as it is commonly asserted that the deleterious character of water is mainly due to the putrefactive processes taking place, which are probably directly proportionate to the quantity of ammonia produced, it is evident that the determination of this compound is of considerable importance. The proportion of albuminous and allied constituents in a sample can, moreover, be measured by the quantity of ammonia produced when the water is boiled with an alkaline solution of potassium permanganate. Upon the foregoing facts, Messrs. Wanklyn, Chapman, and Smith[124] have based a method for the determination of the sanitary quality of potable waters, which is in very general use. It involves, first, an estimation of the ammonia generated upon distilling the water with sodium carbonate (“free” ammonia); second, the quantity given off by boiling with alkaline potassium permanganate (“albuminoid” ammonia). In case the water tested is contaminated with urea, which is not improbable, this compound will be decomposed into ammonia by the treatment with sodium carbonate. The following solutions are employed in the execution of the test:—

Ammonium Chloride.—Dissolve 1·5735 grammes of the dry and pure salt in 1 litre of distilled water. When required for use, dilute 100 c.c. of the solution to 1 litre; 1 c.c. of this diluted solution contains ·00005 gramme of NH3.

Pure Sodium Carbonate.—The ordinary pure reagent is freed from any ammonia possibly contained by heating it in a platinum capsule.

Pure distilled Water.—This is obtained as directed on p. 204.

Nessler’s Reagent.—This is a strong alkaline solution of mercury biniodide. It may be prepared by first dissolving 62·5 grammes of potassium iodide in 250 c.c. of hot distilled water (reserving 10 c.c. of the solution), and adding a concentrated solution of mercury bichloride, with constant shaking, to the remainder, until a permanent precipitate remains undissolved; this is then brought in solution by means of the 10 c.c. of iodide solution, set aside, and the addition of mercury bichloride is carefully continued until a slight precipitate reappears. A concentrated solution of potassium hydroxide (200 grammes dissolved in water) is now added, and the volume of the whole made up with distilled water to 1 litre. The solution is then allowed to subside, after which it is decanted and preserved in a well-stoppered bottle.

Permanganate solution.—Dissolve 8 grammes of potassium permanganate and 200 grammes of potassium hydroxide in 1 litre of water, and boil to expel any ammonia present.

The estimation of free and albuminoid ammonia is made as follows:—100 c.c. of the water to be examined are introduced into a glass retort, which connects with a Liebig’s condenser, and has previously been thoroughly cleansed by boiling with distilled water; one gramme of pure sodium carbonate is added, and the water distilled until 40 c.c. have passed over, the distillate being separately collected in four 10 c.c. cylinders or tubes. About 10 c.c. of the alkaline solution of potassium permanganate is then added to the remaining contents of the retort, and the distillation continued almost to dryness. The second distillate is likewise collected in fractions of 10 c.c. each. It is advisable to so regulate the process of distillation, that only about 10 c.c. pass over in the space of eight minutes. The two sets of distillates are then separately tested by adding 0·5 c.c. of the Nessler solution to each cylinder, well stirring the mixture, and setting it aside for at least five minutes. A series of comparison tubes (10 c.c. in capacity) are prepared by adding ·001, ·003, ·005 up to ·01 gramme of ammonium chloride, and filling to the 10 c.c. mark with pure distilled water; 0·5 c.c. of the Nessler reagent being added to each. The degree of coloration exhibited in the cylinders containing the two sets of distillates is then matched by the comparison cylinders.

It is evident, that from the data thus obtained, the amount of ammonia obtained by the first distillation with sodium carbonate (free ammonia), and by the second distillation with alkaline potassium permanganate (albuminoid ammonia), can be determined. It has been previously mentioned that urea evolves ammonia when boiled with sodium carbonate; the amount of ammonia obtained by the first process of distillation will therefore include that actually contained as such in the water, and that generated by the decomposition of any urea possibly present. As the presence of this body is incompatible with a good drinking water, this fact is of little real importance. In case, however, it be desired to make an estimation of the free ammonia really present, 500 c.c. of the water to be tested are treated with 1 or 2 c.c. of calcium chloride solution, then with a slight excess of potassium hydroxide, and the liquid filtered. It is next distilled as directed above, and the remaining contents of the retort made up to 500 c.c. 200 c.c. of the original sample are then subjected to the same treatment with calcium chloride and potassium hydroxide, and filtered. The second solution, which contains all the ammonia originally present in the water, is now tested with Nessler’s reagent, the solution first obtained by diluting the contents of the retort being employed, instead of pure distilled water, for comparison.

The proportions of free and albuminoid ammonia found in the preceding operations are usually expressed in parts per 100,000 of the water. Wanklyn gives the following amounts of free and albuminoid ammonia contained in 100,000 parts of several kinds of water:—

The same authority makes the following classification of potable water, reference being made to parts of albuminoid ammonia present in 100,000 parts:—

The presence of any considerable proportion of free ammonia is usually indicative of recent sewage contamination. In the absence of free ammonia, a water need not be rejected unless the albuminoid ammonia exceeds 0·010 part, but a water containing over 0·015 part of albuminoid ammonia should be condemned under all circumstances.

6. Nitrogen as nitrites and nitrates.—It is quite generally accepted that the presence in water of the oxidation products of nitrogen, is to be ascribed to the oxidation of nitrogenous organic matter, unless they are the result of percolation through soil containing nitrates, and, for this reason, considerable importance attaches to the quantitative estimation of the nitrogen present in the state of nitrates, and, in some cases, nitrites. One of the most reliable methods for this determination is the eudiometric process of Frankland, which is based upon that of Crum,[125] and consists in agitating the concentrated water with mercury and strong sulphuric acid, and measuring the volume of nitric oxide formed by the reduction of nitrates and nitrites. Owing, however, to the necessity of employing gas apparatus, this method is not in very general use. Wanklyn’s process is the following:—100 c.c. of the sample are made alkaline with pure sodium hydroxide, evaporated to about one-fourth of its original volume, next made up to 100 c.c. by adding pure distilled water, and introduced into a flask which connects with a U-tube filled with powdered glass moistened with hydrochloric acid. A piece of aluminium foil is then added to the contents of the flask, and the mixture is allowed to stand at rest for six or seven hours. The contents of the U-tube are now transferred to the flask, the latter is connected with a Liebig’s condenser and the liquid distilled. The proportion of ammonia contained in the distillate is determined by Nessler’s reagent as previously described, from which the amount of nitrogen present as nitrates and nitrites is calculated.

Griess[126] has suggested a very useful process for the determination of nitrous acid and nitrites in potable waters. It is executed by placing 100 c.c. of the filtered water in a glass cylinder, and adding a few drops of dilute hydrochloric acid, and 1 c.c. of a solution of sulphanilic acid and naphthylamine hydrochloride. In the presence of nitrites, a beautiful rose-red colour (due to the formation of azobenzol-naphthylamine sulphonic acid), will be produced. The proportion of nitrites contained in the water, is ascertained by simultaneously subjecting a solution of potassium nitrite, of known strength, to the same treatment, and matching the degree of colour obtained, as in the Nessler process. This solution can be prepared by dissolving 0·406 gramme of dry silver nitrite in hot water, and adding a slight excess of potassium chloride. After cooling, the solution is made up to one litre, the silver chloride allowed to settle, and the clear liquid filtered. If 100 c.c. of the filtrate are further diluted to one litre, each c.c. will contain 0·00001 gramme of nitrous acid.

In Ditmar’s method, the residue obtained by the evaporation of the water, is first mixed with pure sodium hydroxide, and placed in a small silver boat. It is next introduced into a combustion tube and burned in a current of hydrogen, the evolved gases being received in an absorption apparatus filled with very dilute hydrochloric acid. In this method the amount of ammonia formed, is likewise estimated by means of Nessler’s solution. The proportion of organic nitrogen is found by deducting the free ammonia present in the water and multiplying the remainder by 14/17.

Messrs. DuprÉ and Hake[127] determine the organic carbon in water essentially as follows:—The residue of the evaporation of the water is obtained in a very thin silver dish, which can be rolled up and introduced into a combustion tube filled three-fourths of its length with cupric oxide. The residue is then burned in a stream of oxygen. The evolved carbonic acid is absorbed in a solution of barium hydroxide, the precipitate formed being collected upon a filter, washed, dried, and weighed; its weight, divided by 19·4, gives the amount of organic carbon present in the sample. The carbonates and nitrates originally contained in the water can be removed by boiling with a saturated solution of sulphurous acid before the preliminary evaporation.

Frankland gives the following average proportions of nitrogen, as nitrates, occurring in 100,000 parts of various kinds of water:—

Other authorities regard the presence of more than 0·6 part of nitrogen as nitrates per 100,000 parts of water as indicating dangerous pollution.

At the International Pharmaceutical Congress held in Brussels,[128] the following standards of purity for potable water were recommended:—

1st. A water should be limpid, transparent, colourless, without smell, and free of matter in suspension.

2nd. It should be fresh, with a pleasant taste, and its temperature should not vary much, and certainly not be higher than 15°.

3rd. It should not contain noxious animal or vegetable matter, and especially none of these substances in a state of decomposition.

4th. It should not contain more than 6 to 10 mgrms. of organic matter per litre, expressed in terms of oxalic acid. It should not contain nitrogenous matter.

5th. The nitrogenous organic matter, oxidised with an alkaline solution of potassium permanganate, should not yield more than 0·01 part of albuminoid ammonia per 100,000.

6th. It should not assume a disagreeable smell after having been kept in an open or closed vessel.

7th. It should not contain white algÆ, nor numerous infusoria, bacteria, etc.

8th. It must hold air in solution, which should contain a larger proportion of oxygen than ordinary air.

9th. It should not contain, per litre, more than:—

In the Municipal Laboratory of Paris, the following standards for potable waters are employed. One litre must not contain more than:—

100 c.c. should contain 3·25 c.c. of gas, 10 per cent. of which should be carbonic acid and 33 1/3 per cent. oxygen.

Professor J. W. Mallet[129] suggests the idea, that the noxious character of potable waters containing nitrates and nitrites, with but small proportions of organic matter, may be due to the presence of a special nitrifying ferment belonging to the lower organisms, which are capable of propagating disease.

In regard to the degree of importance that should attach to definite and arbitrary standards of purity, it appears to be accepted that, although the data afforded as the result of chemical tests are often of value in discriminating between pure and impure waters, but little reliance should be placed upon such criteria alone.

Professor Mallet, who has devoted much attention to the investigation of potable waters, and whose opinion on this subject is entitled to the highest consideration, arrived at the following conclusions concerning the more vital points at issue in the determination of the hygienic character of water:—

“1. It is not possible to decide absolutely upon the wholesomeness or unwholesomeness of a drinking water by the mere use of any of the processes examined for the estimation of organic matter or its constituents.

“2. I would even go further, and say that in judging the sanitary character of the water, not only must such processes be used in conjunction with the investigation of other evidence of a more general sort, as to the source and history of the water, but should even be deemed of secondary importance in weighing the reasons for accepting or rejecting a water not manifestly unfit for drinking on other grounds.

“3. There are no sound grounds on which to establish such general ‘standards of purity’ as have been proposed, looking to exact amounts of ‘organic carbon’ or ‘nitrogen,’ ‘albuminoid-ammonia,’ ‘oxygen of permanganate consumed,’ etc., as permissible or not.

“4. Two entirely legitimate directions seem to be open for the useful examination by chemical means of the organic constituents of drinking water, namely; first, the detection of very gross pollution, * * * * and, secondly, the periodical examination of a water supply, as of a great city, in order that the normal or usual character of the water having been previously ascertained, any suspicious changes, which from time to time may occur, shall be promptly detected and their cause investigated.”

The microscopic and biological investigations of water are useful adjuncts to the chemical examination. The former is made by allowing a litre or more of the sample to remain at rest for several hours, collecting the deposit formed and inspecting it by means of the microscope, using low magnifying power at first. It will be found advantageous to stain portions of the sediment obtained with aniline violet, which, by a sort of predilection, attaches itself to particular forms of vegetable and animal life, thereby rendering them more distinct. The matters most usually observed in the microscopic examination of the deposit are:—

1st. Numerous lifeless substances, such as mineral matters, vegetable debris, muscular and cellular tissues, hairs, hemp, wool, cotton, silk, starch cells, insect remains, and pollen grains.

2nd. Living vegetable forms, such as confervÆ, various algÆ, oscillatoria, desmids, diatoms, and bacteria.

3rd. Living animal forms, including many varieties of infusoria and animalcula. Of the latter, those known as “saprophytes” are regarded as specially indicating the presence of sewage contamination.

Certain varieties of bacteria have been found associated with some forms of disease, and particular attention has been bestowed upon the study of these germs. The biological examination of water consists of pathological experiments on living animals, made by injecting a solution of the water-residue beneath the skins of rabbits, etc., and of experiments made by inoculating culture gelatine with the water. Of the latter methods of examination, that originally suggested by Dr. Koch, of Berlin, and described by Dr. Percy F. Frankland,[130] is well worthy of mention. In this process, the lower forms of life are cultivated in a solid medium, by means of which the growth of each colony is localised and rendered suitable for microscopic inspection.

The medium employed by Dr. Frankland has the following composition:

The finely-cut meat is first infused in half a litre of cold water for two hours and strained; the gelatine is digested in the other half-litre of water, then mixed with the meat-extract, and the whole heated until the gelatine is completely dissolved, when the peptone and salt are added.

The liquid is now cautiously neutralised with sodium carbonate, and clarified by beating it together with two or three eggs, boiling, straining through cloth, and filtering hot through bibulous paper; upon cooling it sets to a transparent jelly. Before setting, 7 c.c. of the liquid are introduced into a series of clean test-tubes, which are tightly plugged with cotton-wool and then sterilised by steaming them half-an-hour for three or four consecutive days. It is necessary to observe special precautions in the collection of the sample of water to be examined. Glass-stoppered bottles are well adapted for this purpose. These are to be very thoroughly washed with distilled water, then dried and finally sterilised by heating in an air-bath for three or four hours at a temperature of from 150° to 180°.

The actual examination of the water is executed by first heating one of the test-tubes containing the sterilised gelatin medium in a water-bath to 30°, by which it is fused. The external portion of the cotton-wool is next burned, the tube opened, and a certain number of drops of the water to be tested (previously well shaken) are introduced by means of a sterilised pipette. The mixture is immediately poured out upon a clean and sterilised glass plate which rests in a perfectly horizontal position, and is covered by a glass shade. The plate is supported by a glass tripod which dips into a dish containing a two per cent. solution of mercuric chloride—thus forming an antiseptic protection from the external air. The tripods, dishes, etc., are sterilised by washing them with the mercuric chloride solution. As soon as the gelatine mixture has set, the glass plate (together with the cover) is introduced into an air-bath kept at a temperature of from 20°-25°, where it is allowed to remain for two to five days for incubation. The individual organisms and the progress of the formation of colonies are observed from time to time by inspecting the plate, which can be done without removing the glass cover. As soon as they have become easily visible to the naked eye, the plate is removed from the bath, and placed upon another glass plate, which is ruled in squares, and put over a black paper. The colonies are then counted by aid of a lens, or, if they are too numerous to admit of this, the number contained in a few of the squares is determined and multiplied accordingly.

Dr. Frankland has applied the foregoing method to the examination of the London water supply (1885), with the following results:—

Micro-organisms in 1 c.c.

In Plate XI., Fig. 1 exhibits the animal and vegetable living forms contained in Croton water. They have been catalogued as follows:—

(a) Asterionella formosa, vegetable; a diatom, × 312.
(b) Pediastrum simplex, vegetable; a desmid, × 200.
(c) Cyclotella astrÆa, vegetable; a diatom, × 200.
(d) Vorticella; an animalcule, × 312.
(e) Conferva, vegetable; “green scum,” × 40.
(f) Epithelial cell; × 200.
(g) Fragillaria cupucina, vegetable; a diatom, × 200.
(h) Heteromita ovata; an animalcule, × 500.
(i) Halteria grandinella (?); an animalcule, × 200.
(k) Anguillula fluviatilis; a water-worm, × 312.
(l) Amoeba porrecta; an animalcule, × 200.
(n) Dinophrys; an animalcule, × 200.
(o) Didymoprium borreri, vegetable; a desmid, × 200.
(p) Tabellaria fenestrata, vegetable; a diatom, × 312.
(q) Free vorticella; an animalcule, × 200.
(r) Coccudina costata, dividing; an animalcule, × 312.
(s) Monas umbra; an animalcule, × 312.
(t) Cyclidium obscissum; an animalcule, × 312.
(u) Chilodon cucullulus; an animalcule, × 200.
(v) Epistylis nutans; young animalcules, × 200.
(w) Paramecium; an animalcule, × 200.
(x) Difflugia striolata, the lorica or case; an animalcule, × 200.
(y) Conferva; vegetable “green scum,” × 312.
(z) Vorticella microstoma; an animalcule, × 200.
(aa) Fragments of dyed wood, × 200.
(cc) Gomphonema acuminatum, vegetable; a diatom, × 200.
(ee) Arthrodesmus octocornis, vegetable; a desmid, × 312.
(ff) Scenodesmus quadricauda, vegetable; a desmid, × 200.
(ii) Navicula rhynchocephala (?), vegetable; a diatom, × 200.

Fig. 2 represents the organisms found in the Brooklyn (Ridgwood) water supply:—

(a) Actinophrys sol; an animalcule, × 200.
(b) Coccudina costata; an animalcule, × 200.
(c) ChÆtonotus squamatus; hairy-backed animalcule, × 200.
(d) Notommata; a rotiferous animalcule, × 200.
(e) Amoeba guttula; an animalcule, × 200.
(f) Melosira orichalaea, vegetable; a diatom, × 200.
(g) Vorticella microstoma; animalcules, × 200.
(h) ChÆtonotus larus; hairy-backed animalcule, × 200.
(i) Tabellaria flocculosa, vegetable; a diatom, × 200.

The original drawings from which Plate XI. is taken were prepared by Mr. William B. Lewis, for the Metropolitan Board of Health.

The presence of these organisms, however startling some of them may be in appearance, is usually not objectionable; indeed, microscopic vegetable growths are frequently of service in the purification of potable water. The more important forms of bacteria (bacilli, etc.), present minute rod-like shapes, far less impressive in appearance.

Considerable difference of opinion exists in regard to the sanitary value of the results afforded by the biological examination of water. While the number of bacteria found in a given quantity of water may be of aid in the formation of an opinion as to its relative safety for domestic purposes, it should be borne in mind that these micro-organisms are almost omnipresent, being contained in the air, and in soils, and articles of food.

The following tabulation shows the relative purity of the water supply of several American cities, as determined by Prof. A. R. Leeds, in June, 1881:—

Cl. signifies clear. Sl. Tb., slightly turbid. Tb., turbidity somewhat more marked.