CHEMICAL NOTES.

Note I.—Hydrocyanic or Prussic Acid.

Nature of—Strength of different preparations of, English and foreign—Where found—Tests, preliminary: (1) Odour—(2) Silver—(3) Prussian blue—(4) Sulphur—(5) Guaiacum—(6) Uranium—(7) Picric acid—(8) Cupric sulphate—(9) Cobalt chloride—(10) Mercuric oxide—(11) Peroxide of hydrogen—(12) Mercurous nitrate. Test apparatus—Salts of hydrocyanic acid: (1) Potassium cyanide—(2) Mercuric cyanide—(3) Cyanides of the heavy metals—(4) Double cyanides—(5) Sulphocyanides—Oil of bitter almonds—Antidotes—Fatal dose—Symptoms—Post-mortem appearances—Drops and minims—Period after death at which hydrocyanic acid can be discovered—Formic acid to be tested for—Processes.

Synonyms.—Cyanhydric or prussic acid, Hydric cyanide, Hydrogen cyanide, Acidum borussicum, BlausaÜre, BerlinerblausaÜre.

Formula HCN, i.e., a compound of single atoms, of hydrogen, carbon, and nitrogen, in the proportions by weight of 1 + 12 + 14 = 27. In its pure state (anhydrous, or free from water), it is a feebly acid, colourless, mobile liquid, inflammable and very volatile. Boiling point 24·5° C. Much lighter than water: sp. gr. ·7058. It has a characteristic overpowering and oppressive odour, resembling peach-blossom or laurel-water. But the anhydrous acid, from its volatility and dangerous character is rarely seen or made. In commerce it is always found as a dilute aqueous solution, the varying strengths in real HCN being:—

In this country, only Scheele’s, and the British Pharmacopoeia (2 per cent.) acid, are usually met with.

These numbers, however, must be regarded merely as rough approximations for two reasons; first, on account of the extreme volatility of the acid—if loosely stoppered, or frequently opened, it rapidly loses strength—second, both the anhydrous acid and its aqueous solution are decomposed by light, with formation of a brown matter. This change is supposed to be retarded by a trace of mineral acid, hence a little hydrochloric or sulphuric acid is frequently added to the commercial solution with this object. But the acid may even be stronger than supposed, as the methods of preparation are somewhat various, and the one adopted may have been carelessly carried out. Scheele’s acid is said to be the most popular among medical men; samples of it obtained from different large firms and examined by the author showed very irregular strengths, the lowest being 2, and the highest 8 per cent. The latter was purchased at the shop of a chemist who said he had made it himself, and could guarantee it was of full strength. He had evidently made allowance for deterioration. Woodman and Tidy found 16 samples sold in one neighbourhood as B.P. acid to contain 0·6 to 3·2 per cent. of HCN; others have found 0·25 per cent. not infrequent. It follows that if, in a poison case, a bottle has been found of a hydrocyanic preparation of a definite name, or even with a certain strength or dose marked on it, it will not be safe to trust to such figures without actually determining the amount. In Ball’s trial (Lewes, 1860), the judge asked whether this variation in strength would not make the difference between a medicinal and a poisonous dose? It would not, as the maximum medicinal dose, 4 grains, of even the abnormally strong (8 per cent.) Scheele’s acid mentioned above, would only contain 0·32 grain of anhydrous HCN, and it requires at least half a grain to cause death, while about 1 grain is the usual fatal quantity. And a medical man would not even give the maximum medicinal dose as a beginning, and without precaution.

ACIDUM HYDROCYANICUM DILUTUM, PHARMACOPŒIA BRITANNICA.

We shall use the abbreviation “B. P. 2 per cent.” for this acid, which has the characteristic odour, a sp. gr. of ·997, and a taste “at first bland and sweet, ultimately pungent and acrid” (Thomson), “hot and bitter” (Taylor), “cooling, with pungent bitter aftertaste” (Watts). If pure, it only slightly and transiently reddens litmus; if other acids have been added to keep it, it may have a stronger reddening effect. Also, if pure, it leaves no residue on platinum, and gives no precipitate with barium chloride, but with silver nitrate it gives an immediate white curdy precipitate of silver cyanide, not blackening in daylight as the chloride does, soluble in ammonia, insoluble in dilute, but soluble in hot concentrated nitric acid. It dissolves mercuric oxide, giving a mercuric cyanide which may be obtained in white crystals on evaporation. The vapour is said to be more deadly than the fluid acid. The weaker the acid, the more permanent it is. Glycerine increases its stability (J. Williams); this might be useful if suspected substances had to be kept a long time.

Occurrence.—Hydrocyanic acid itself has never been found as a natural constituent of the body, although a compound of cyanogen occurs in the saliva (see Sulphocyanides). Hydrocyanic acid is not formed during putrefaction, nor by heating organic substances with chemical reagents at temperatures up to 212° F., as in testing for poisons. The only way in which it may be generated from animal matter is by heating with alkalies to a red heat;[20] this cannot, of course, happen in the ordinary process of testing for prussic acid, though it must be remembered that cyanide might thus be formed in an ash (by burning), without having been present in the original substance.

It is rather frequent, however, in the vegetable kingdom, and consequently in a poisoning case the defence often sets up the theory that it has been ingested in the food (Tawell’s Trial, &c.). It is necessary, therefore, to examine in what kind of food, and to what amount, it may be taken.

Its principal source is the seeds, leaves, and flowers, and sometimes the bark, of most of the species of the sub-orders AmygdaleÆ and PomeÆ of the natural order RosaceÆ. It does not occur in them ready-formed. There is a substance called Amygdalin, a white bitterish crystalline body, which may be extracted by alcohol from these plants. Amygdalin when dissolved by itself in water does not produce HCN, and is probably harmless, but there exists by its side in the plant a species of ferment called Emulsin or Synaptase, which has the power, when macerated in water with amygdalin, of breaking up the latter into glucose (so-called grape-sugar), benzoyl hydride (oil of bitter almonds), and hydrocyanic acid. In the plant the amygdalin apparently exists in cells apart from the emulsin, but by crushing in water, or masticating in the mouth, the change is very rapidly effected. By long soaking the same result may happen, as in cherry brandy; here the diluted spirit dissolves the amygdalin, and the emulsin then may act. But if, in the stomach, the apple-pips or cherry-stones should be found whole, it is almost impossible that the amygdalin should be decomposed, protected as it is by its horny or stony envelope. Stones and pips, in fact, pass through the body intact, and are found in the fÆces.

Yet as amygdalin and its decomposition may be much mentioned by the defence, the following account may be useful.

100 parts of amygdalin yield 6 parts HCN.

It has been found in the species of RosaceÆ given below, generally in fruit, flowers, leaves, sometimes bark, rarely root.

Pyrus malus (apple pips), domesticus (pear).

Prunus spinosa (sloe), avium (bird cherry), padus (wild service), Virginiana or serotina (wild black cherry), capricida, insititia (bullace), domestica (plum, damson, &c.).

Amygdalus communis (almond), Persica (peach), lÆvis (nectarine).

Armeniaca vulgaris (apricot).

Cerasus communis (cherry), acida, laurocerasus (cherry-laurel), Lusitanica (Portugal laurel).

Cydonia vulgaris (quince).

Sorbus aucuparia (mountain ash), torminalis, hybrida.

CratÆgus oxyacantha (hawthorn, young branches).

SpirÆa aruncus, sorbifolia, japonica (not in herbaceous species).

Hydrocyanic acid, ready formed, has been found in the roots of the bitter and sweet cassava (Jatropha manihot).

If the poisonous dose of the B. P. (2 per cent.) acid be at least 30 minims (Royle’s Mat. Med., Dr. Harley, 6th ed.) the following table shows the amount of some of the above which is needed.

Sweet almonds contain emulsin, but no amygdalin, hence give no HCN (see Tawell’s Trial, p. 40).

According to my own experiments, 837 sweet apples (apples weighing 135 pounds, pips about 5 oz.), would be required for a poisonous dose of HCN; whereas 130 bitter apples, weighing 18 pounds, and the pips about 2½ oz., would suffice. The pips of bitter apples are bigger, more numerous, and weigh about three times as much as those of sweet apples.

Among substances containing much more HCN, and actually poisonous on that account, are:—

(Allen, Comm. Org. Anal.) It is obvious that of fruits an impossibly large quantity must be eaten to produce any considerable amount of HCN. In Tawell’s trial, Mr. Cooper, the analyst, deposed that the seeds from 15 apples gave him an exceedingly small quantity of Prussian blue. Whereas, Henry Thomas, a druggist’s assistant, stated that “15 small apples gave 2¼ grains of silver cyanide” [equal to 0·46, or nearly ½ a grain, of anhydrous HCN, corresponding to 25 minims of B. P. acid, nearly a poisonous dose!] “This was done under the direction of a lecturer at the London Hospital.” A fair sample of the erroneous and bewildering evidence that is frequently offered in courts of justice.

Mr. Cooper also stated “there is a great difference between bitter and sweet apples; the bitter contain a great deal of prussic acid, the sweet, I believe, none at all!” This statement is misleading; no apples contain prussic acid, but all that I have met with will yield it by maceration, as all contain amygdalin. The highest class of eating apples, such as Newtown pippins, Ribstones, and Blenheims, contain only a minute trace. These have very few pips, 3 to 5 to each apple, while the bitter varieties, such as “winesours,” have 9 to 13 pips.

In the arts, cyanides are used in photography, dyeing, cleaning lace and metals, electro-plating, removing silver stains, &c. Their solutions may cause accidental poisoning, either by the fumes or by absorption through the skin, especially if the latter is abraded.

Hydrocyanic acid is also formed (1) in the preparation of nitrous ether (sweet spirit of nitre), (2) by distilling albumen, fibrin, casein, or gelatin, with sulphuric acid and bichromate of potash, or manganese peroxide, (3) by the dry distillation of albuminous bodies. It is hardly necessary to say that these formations could not occur in the ordinary methods of testing.

Tests: Preliminary.—It cannot be too strongly insisted that all operations for the detection of HCN should be carried out as soon after death as possible, on account of the loss from volatility, or from secondary changes. (See Sulphocyanides.)

Allen asserts (Commercial Organic Analysis, 1879), that detection in the body is rarely possible more than twenty-four hours after death; but Taylor (Med. Juris., 1873, p. 368) has found it in the stomach twelve days after, saying, however, that “after the stomach had been exposed a few days longer, all had disappeared.” In a dog’s stomach he found it, after twenty-four hours’ exposure, and washing with water. In a human stomach, success was achieved seven days after death, where no odour was perceptible; in another case, after twenty-two days in the stomach, and after two months in the spleen. It may be found in the stomach, and not in the tissues; but in most cases it is easily detected, soon after death, in the blood, organs, &c. The vapour of HCN will traverse paper, wet or dry bladder, &c., in a few minutes (Taylor), and few stoppers are close enough to retain it. Hence care should be taken to shut up the suspected matters at once in glass bottles accurately stoppered; bad stoppers are worse than corks.

The stomach should be first examined entire, to ascertain odour, &c., noticing whether alkaline or acid, then cut in pieces, under distilled water sufficient to cover it, the whole measured, and one-half (acidulated with tartaric acid, if alkaline), placed in a capacious retort, and distilled in a bath of water saturated with salt to raise the boiling point. The condenser should be well supplied with cold water, the receiver attached airtight, with a mercury valve (a narrow glass U-tube, containing mercury), to prevent undue pressure. A little distilled water, about ½ oz., should be placed in the receiver. The distillation should be continued till one-third to one-half of the original liquid has passed over. The tests may then be applied to the distillate.

Allen recommends us to distil with water alone about one-half. If there is no result on testing the distillate, continue with addition of tartaric acid. Finally, add a considerable excess of moderately dilute sulphuric and hydrochloric acid, and carry the distillation nearly to dryness. In the last stage sulpho-, ferro-and ferricyanides and mercuric cyanide are decomposed, and give HCN. The original should be tested for ferrocyanide, &c. This seems a process calculated to give the clearest idea of the form in which the HCN is present, but is open to the objection that it is protracted, and may hence cause loss.

Sokoloff. (Chem. Centr., 1876, 603) advises a much more heroic treatment. “Strongly acidify with sulphuric acid, and distil over a water bath for two or three days, replenishing the water as evaporated. The longer the distillation, the more accurate the result.” He adds, that the muscles contain the greater part of the HCN. He quotes figures in support of his results, but I have not found such prolongation necessary; and we must remember that HCN is decomposed by heating with moderately strong mineral acids.

The following modification, proposed by the author, may be advantageous, as diminishing the risk of loss, and also effecting concentration:—Prepare exactly equivalent solutions of silver nitrate, and hydrochloric acid: the silver solution may contain 17 grammes of silver nitrate, the hydrochloric solution 3·65 grammes of hydric chloride, per litre. Place in the receiver 100 cubic centimetres of the silver solution (= 1·70 gramme silver nitrate) before distillation. This is allowing large excess, to provide for exceptional quantities of HCN. If any quantity of HCN be present, the liquid in the receiver will become milky; if it does not, there cannot be more than a minute trace. Transfer the distillate and washings to a retort, provided with a thistle-funnel, and boil down to one-third of its bulk; then add, through the funnel, 100 cubic centimetres of the hydric chloride solution, which will precipitate all the silver as chloride, and liberate the HCN. Distill with the same precautions as before: the first 25 cubic centimetres will contain probably all the HCN. If doubted, a further quantity may be collected and tested. The 25 cubic centimetres of distillate may now be subjected to the following tests, taking care that each portion is measured before being examined, in order that the idea of the quantity present may be definite. For instance, in the Prussian blue, and sulphocyanide tests, the resulting colour may be imitated by standard solutions: in the silver test, a standard silver solution should also be used, and thus a triply-confirmed knowledge of the quantity present may be attained; and little bottles, containing the results, should be preserved, to show in the courts of justice.

I. Odour.—All tests involving odour are affected seriously by the remarkable differences between different people as to their sense of smell. We hear much of “colour-blindness;” but the analogous olfactory defect has almost escaped remark. Yet “smell-blindness,” as I have formerly christened it, or “anozism,” if a Greek word be required, is exceedingly common, and chemists and medical men are frequently afflicted with it. I have known an artist, who could not smell strong ammonia, yet delighted in the odour of new paint, which he compared to roses. Many laboratory students can neither smell acetic acid, arseniuretted hydrogen, nor cyanogen. An assistant was so fond of sulphuretted hydrogen, that he was once found insensible beside the apparatus, having narcotized himself with the gas (he recovered); and many more such eccentricities. In the case of prussic acid these diversities are enormous. Some are so sensitive, that the least trace in a room becomes rapidly unbearable, causing headache and nausea; others are like photographers, and can work in a heavily-cyanogened atmosphere. Such idiosyncrasies become of great importance in evidence; for example:—

In Tawell’s trial, Mr. Champneys, surgeon, testified as follows: “Have no experience in detecting odour of prussic acid in a human subject. Should think it may be taken without detection. Should expect it in the mouth and breath, but there may be exceptions. There was no odour in her [the deceased’s] breath; but, on opening the body, I was positive I smelt prussic acid. The other two surgeons could not smell it.” Afterwards, when the contents of the stomach were transferred to a jar, neither the three surgeons, nor Mr. Cooper, the analyst, could perceive the least odour of prussic acid, even when the contents were boiled. Nor was it smelt in the blood. Mr. Cooper subsequently stated: “I have no doubt that prussic acid may exist without being smelt: absence of smell may arise from dilution, or from its being covered by the smell of other substances. When I smell it, it affects spasmodically the back of the throat. Sometimes it has produced a spasmodic constriction about the throat without my smelling it.” Here was a well-marked case of intermittent smell-blindness.

There were also several questions as to whether prussic acid might have existed in the form of an inodorous salt. Mr. Champneys further stated that he put ½ drachm of prussic acid into a tumbler filled with Guinness’s porter, and the smell was scarcely perceptible. Mr. Norblad, surgeon, deposed that he mixed 12 grains of prussic acid with a pint of porter, but could not then smell it. “Some of the porter dropped on the table, and I did then smell it.” In the same trial, Henry Thomas, druggist’s assistant, mixed 30 drops of B.P. prussic acid with 11 oz. of porter, and found the odour of the acid slightly perceptible; yet, when he was pouring Scheele’s acid from a bottle, three women had to leave the room to avoid suffocation!

In a case of suicide by cyanide of potassium (Chem. News, 1861, p. 261), the smell of prussic acid was not perceived by the surgeon, either immediately after death or at the post-mortem examination, nor by the analyst until the contents had been distilled with dilute sulphuric acid.

To help in elucidating this matter I have made some experiments as to the detection of the odour of prussic acid. An acid of 2 per cent. strength (B.P.) was used.

1. From a bottle of Guinness’s stout, freshly opened, 3 samples of 1 fluid oz. each were measured. To the first 1 drop of the acid was added, to the second 2 drops, the third being left untouched. This was done out of my sight in another room. They were then privately marked by an assistant, and brought in; when myself and two others, one of them entirely inexperienced, independently and at once classified them without hesitation correctly as to the relative amounts of prussic acid. The odour was so distinct as to produce, when inhaled, a feeling of oppression, and to quite overpower the odour of the beer.

About ¹/₃₀ of a poisonous dose. Hence if a poisonous dose were put into a pint and a half of stout, the odour would be distinct.

2. One drop of the dilute (2 per cent.) acid was added to 6 oz. stout: there resulted a slight but distinct odour of prussic acid. Hence a poisonous dose in nine pints would be smelt. Covered by a watch-glass, with a drop of yellow ammonium sulphide on it, it was warmed; the drop, on evaporation, gave a distinct red sulphocyanide reaction with ferric chloride. Exposed to the air for twenty-four hours, all the above samples had lost their odour, and failed to give the sulphocyanide reaction.

3. Two samples of urine, measuring ½ pint each, were treated respectively with 1 and 2 drops of B.P. acid (strength in this case 1·18 per cent. HCN), and a third ½ pint left untouched, the same precautions being used as with the above beers. Three independent witnesses again classified them without difficulty as to relative amounts of the poison. This is 2 drops of an exceptionally weak acid to the pint.

4. The contents of a human stomach, very fetid from decomposition, were divided into two portions of about 2 oz. each: one was left untouched; to the other 1 grain of mercuric cyanide was added, and then about 5 drops of hydrochloric acid, and a little zinc dust. The whole was well stirred, and shut up close. Next day the odour of HCN was very prominent in the one to which the cyanide had been added, in spite of the strong original smell of both.

5. I cannot agree with Taylor that either peppermint or tobacco mask the odour appreciably.

The odour of nitrobenzol, being similar to that of bitter almonds, might lead to a suspicion of prussic acid without due caution (Woodman and Tidy).

In putrefying, organic matters often develope ammonium sulphide, becoming alkaline. The ammonium sulphide would combine with the HCN to form sulphocyanide of ammonium, which is inodorous, but, by distillation with acids, gives HCN. Sulphocyanide, however, could not be produced unless the original matters were alkaline. In Tawell’s, and most other trials, the stomach contents were acid, as they always are naturally from the gastric juice.

Taylor (Med. Jurisprudence, 1873, vol. i., p. 364) mentions a case where the blood had a strong odour of prussic acid, and the mucous membrane of the stomach, even after it had been washed three times with water, also exhaled a strong odour. In another case (Med. Gaz., vol. xxxvi., p. 104), where 20 grains of Scheele’s acid had been taken with ultimate recovery, the vomited matters had no odour, “showing that, if not concealed by other odours, the whole of the acid must have been absorbed.” Many other instances might be quoted where nothing was smelt, and yet the tests revealed prussic acid.

As to the question about the salts of prussic acid, it may be generally said that all poisonous cyanides would smell in the stomach, except, perhaps, mercuric cyanide. See “Properties of the Salts,” p. 73. Possibly the formation of mercuric cyanide may have accounted for the absence of odour in some of the above cases, as I do not find that mercury was tested for, though its compounds are common medicines. Otherwise it is hardly possible that hydrocyanic poisoning should have been effected, and the acid be still there, without its very characteristic odour being perceptible to an observer with an acute olfactory sense. I have entered at some length into the question of odour, as much importance has been attached to it in the trials, and I still consider it as one of the most delicate and positive of tests.

II. Silver Test.—When silver nitrate is added to a solution containing HCN or a cyanide acidulated with nitric acid, a white precipitate falls of silver cyanide, soluble in ammonia, insoluble in dilute, but soluble in hot concentrated nitric acid, and not blackened by light. This reaction is rendered quantitative according to Liebig’s volumetric method. The original solution is made slightly alkaline by potash, and a standard solution containing 1·7 grammes of silver nitrate per litre (1 cub. centimetre = ·0017 grm. AgNO₃) is added until a permanent white turbidity is produced, seen best over a sheet of black paper or a black book. Then each cub. centimetre used is equivalent to a double quantity or ·00054 grammes of HCN. Formic acid, or chlorides, do not interfere; in fact, it is advantageous to have a little chloride present.

The silver cyanide may be also estimated gravimetrically by adding excess of silver nitrate, collecting the precipitate on a weighed filter, washing, drying, and weighing. Silver cyanide corresponds to two-tenths of its weight of HCN (134 gives 27). If chlorides be present, the mixed precipitate of silver cyanide and chloride is weighed, treated with dilute hydrochloric acid, and weighed again. The HCN is thus displaced, and passes into the filtrate; the silver precipitate, now all as chloride, is weighed again: then the increase of weight multiplied by 27 and divided by 9·5 (the difference between the equivalent weights of silver chloride and cyanide) is equal to the weight of HCN present. But the volumetric process is quite as accurate, and more expeditious.

In poisoning cases advantage is taken of the opacity of silver cyanide thus: A drop of moderately dilute silver nitrate is placed on a watch-glass over the substance, which may be gently warmed, taking care that the steam condensed does not cause the drop to fall. If HCN be present, the drop will become opaque-white from formation of silver cyanide. ¹/₁₀₀ grain of HCN, equal to ¼ grain of B.P. acid, will give this reaction (Taylor). If there is only a small amount, and the action is gradual, the drop on drying in the air may exhibit crystals of silver cyanide, recognizable under the microscope as minute prisms obliquely truncated. Of course the silver nitrate itself may give crystals, but they will be very soluble in water.

Cyanide of silver is decomposed by (1) hydrochloric acid, giving silver chloride; (2) dilute sulphuric acid and zinc, giving silver; (3) sulphuretted hydrogen, giving silver sulphide; in each case HCN is liberated and may be distilled off: then the other tests may be applied.

If sulphuretted hydrogen be present, it will give a black with silver nitrate. The liquid should in this case be previously shaken with just enough carbonate of lead to remove the sulphuretted hydrogen. The latter, however, does not interfere with the Prussian blue or sulphur tests.

When sufficient in quantity, the cyanide of silver, thoroughly dried in a water-bath, may be transferred to a small bulb-tube and heated, the end being closed with the finger. It breaks up into cyanogen gas, silver, and paracyanide of silver, a peculiar glow and effervescence occurring as it decomposes. The cyanogen will have the characteristic bitter almond odour, and, on removing the finger, will burn with a flame violet on the margin and rosy in the centre.

III. Prussian Blue (Scheele).—Add to the solution or distillate caustic potash in excess, then a drop or two of fresh ferrous sulphate (protosulphate of iron), and a little ferric chloride (perchloride of iron—the tinct. ferri perchlor. of the Pharmacopoeia will do), warm gently for a few minutes, add dilute hydrochloric acid in slight excess: if much HCN be present, a deep blue precipitate (Prussian blue) will remain; if only a trace, the liquid will be greenish, and on standing till the next day a blue deposit will form.[22] This is the only blue iron precipitate which is insoluble in dilute hydrochloric acid.[23]

Remarks.—Sulphuretted hydrogen does not interfere with this test, as the black ferrous sulphide dissolves in hydrochloric acid. The amount of iron salts added should have some relation to the amount of HCN present, an idea of which will have been attained by the silver test. Moderate excess of potash must be present all the time till the hydrochloric acid is added. A large amount of iron salt is objectionable, as the yellow colour interferes with the final green tint with traces of HCN. The test may be made quantitative by imitating the tint with a weak standard solution of potassium ferrocyanide treated with a drop of hydrochloric acid and a drop of ferric chloride, on the same principle as “Nesslerizing” (see Wanklyn’s Water Analysis). Finally the precipitate of Prussian blue should be preserved to exhibit at the trial, as this is the most positive, though not the most delicate, test.

IV. Sulphur Test (Liebig).—The liquid to be examined is placed in a somewhat shallow glass dish or beaker, covered almost airtight with a watch-glass, moistened on the under surface with a drop or two of yellow ammonium sulphide. [The ordinary sulphide is commonly yellow enough for the purpose, or, if not, can be made so by warming with a little flowers of sulphur.] After warming gently for a short time (the periods recommended by different authorities vary from half a minute to ten minutes), great care being taken that the steam does not condense and cause the solution on the watch-glass to drop back into the liquid, the cover is removed, dried on a water bath to drive off any excess of ammonium sulphide, treated with a drop or two of water, and a drop of not too acid ferric chloride free from nitric acid and diluted till nearly free from colour. If HCN be present, it will have formed sulphocyanide with the ammonium sulphide, and will therefore generate a blood-red colour with the ferric chloride. If a colour be produced, continue the addition of ferric chloride till no further deepening occurs. The reaction is made quantitative by comparing the tint with that produced by a known quantity of sulphocyanide and ferric solution (Herapath). But there are difficulties in making it exact.

This is the most delicate test for HCN, detecting ¹/_7930th of a grain of HCN in a very dilute liquid, whereas Prussian blue does not discover less than ¹/_780th of a grain (Taylor, Ann. Ch. Pharm. lxv., 263). Salts of acetic, formic, and meconic acids give red colours with ferric chloride, but (1) meconic acid is not volatile; (2) the red from acetic and formic acids is at once removed by a slight excess of dilute hydrochloric acid, sulphocyanide is not; (3) sulphocyanide-red is destroyed by solution of mercuric chloride, the others are not.

The above tests are sufficient, but the following additional ones have been at different times proposed.

V. Guaiacum Test.—Paper dipped in fresh tincture of guaiacum, containing about 3 per cent. of the resin, then dried, then moistened with dilute cupric sulphate solution (2 per cent.), becomes blue in HCN vapour. But the same effect is produced without HCN by almost all oxydants, such as chlorine, bromine, or iodine, ferric chloride, nitric and nitrous acids, chromic acid, peroxide of hydrogen, ozone (Mohr’s Toxicologie), also by ammonia, hypochlorous acid, soluble chromates, &c. (Blyth).

VI. Uranium Test.—A grain or two of pure ferrous salt (ammonio-ferrous sulphate will do), and the same quantity of uranium nitrate, are dissolved in half an ounce of water. Two or three drops of this are placed on a white plate, and a drop of the suspected liquid added. A purple precipitate, or a greyish purple colour in weak solutions, indicates HCN. Cobalt nitrate may be used instead of the uranium salt, and is nearly as delicate. (Carey Lea, American J. of Science [3] ix., 121.)

VII. A hot solution of potassium cyanide mixed with picric acid gives a deep blood-red —“picrocyanic” acid). Free HCN does not give this reaction, and therefore must first be neutralized by an alkali. Said to be more delicate than the iron tests. (C. D. Braun, Zeitschr. f. anal. Ch. iii., 464.)

VIII. Slightly alkalize the distillate with potash, add a few drops of cupric sulphate, and afterwards just enough hydrochloric acid to dissolve the excess of cupric hydrate: white cuprous cyanide will remain undissolved. “This test will detect ¹/₂₀₀₀₀ of HCN in solution.” (Lassaigne, Ann. de Chimie, xxvii., 200.) But a similar effect is produced by hydriodic acid, and potassium iodide might have been administered.

IX. Mix the HCN with excess of alkali, add cobalt chloride and tartaric acid: on exposure to air a deep brown-red colour will be produced. (C. D. Braun, loc. cit.).

X. If to a solution of HCN, potash be added in excess, and then a little very finely pulverised, or precipitated, mercuric oxide, the latter will dissolve. Mercuric oxide is soluble in alkaline fluids only in presence of HCN. (Fresenius, Qual. Anal.).

XI. “With peroxide of hydrogen, natural blood gives effervescence from escape of oxygen, but no discoloration. Blood containing HCN gives a brown colour, the spectroscopic bands disappearing, and no effervescence.” (SchÖnbein.) HÆmatocrystallin, the colouring matter of the blood corpuscles, combines, in fact, with HCN, giving a dark coloured compound which appears to be crystallizable and definite in composition (Hoppe Seyler), does not act as a carrier of oxygen like the natural hÆmatocrystallin, and possesses a distinct spectrum (see Thudichum, Chem. Physiology). The blue masses in the blood described by Ralph (Journ. Microsc. Science, Oct. 24, 1866) have not been found by others.

XII. Mercurous nitrate gives at once with HCN solutions, a black deposit of metallic mercury, and a solution of mercuric cyanide. With calomel, a similar reaction takes place according to Allen, but I have found that the solution is not deodorized even by large excess of calomel, the odour becoming stronger and more pungent than the original HCN. On evaporating, mercuric chloride is left. Probably some cyanogen chloride is formed. The odour is so much intensified that it might be of use as a test. In view of the possible administration of calomel, the reaction is interesting.

Of course it will not be necessary to employ all these methods. The odour, and the silver, Prussian blue, and “sulphur” tests will be sufficient. I would suggest a form of apparatus by which all the latter could be obtained from the original substance without distillation in a retort.

A shallow beaker or glass jar is closed by an india-rubber stopper, through two holes in which are passed glass rods ending in glass spoon bowls bent at right angles, so as to be horizontal when mounted. The bowls should be one inch in diameter, and will have to be specially made. In the first bowl a few drops of silver nitrate are placed, in the second a little potash. The apparatus is put in a warm place for six or eight hours, then the two rods are removed, a third rod substituted, its bowl containing a drop or two of yellow ammonium sulphide, the other hole plugged, and the apparatus put back in the warm place for two or three hours more. The first bowl will have the silver cyanide, the second should be treated with ferric and ferrous salt and hydrochloric acid for Prussian blue (vide), the third evaporated and ferric chloride added for the sulphocyanide test. This arrangement prevents loss of HCN by volatilization, and also, with a little care, avoids any danger of the reagent dropping back into the solution. The three rods cannot safely be placed in together, as the sulphide vapour would blacken the silver.

For the modifications in testing necessitated by the presence of mercury, &c., see under the different Salts.

SALTS.

Hydrocyanic acid combines with bases to form the cyanides, which may be thus grouped:—

A. Cyanides of the Alkalies (potassium, sodium, ammonium), and of the Alkaline Earths (barium, strontium, calcium, magnesium). These are all soluble in water, are alkaline to test paper, and are decomposed by all acids, even carbonic, hence they exhale an odour of HCN, and are nearly as poisonous as prussic acid itself. If they are present, the stomach contents must be alkaline. The only member of this group likely to be met with is

Potassium Cyanide, KCN. Broken opaque white lumps, or small crystals, deliquescent, smelling strongly of HCN, soapy to the feel, often containing much carbonate, and therefore effervescing with acids, easily fused by heat to a clear liquid, very soluble in water, less in alcohol. Used for removing silver stains in the form of “cyanogen soap,” but very dangerous, as a cut or scratch may cause absorption, and even the unbroken skin, according to Allen, may absorb enough to cause symptoms. Its aqueous solution decomposes spontaneously into formiate of potassium, ammonia, and a brown substance. Its taste is bitter and acrid, causing constriction and a burning heat in the throat. It is very strongly alkaline. Distilled with dilute acids it gives off all its HCN. It easily responds to the other tests. In a case of poisoning investigated by Dr. Bernays, a piece of potassium cyanide was found in the deceased’s mouth, which was much inflamed by its acridity. The alkali being strong, and the acid weak, cyanide of potassium has most of the effects of an alkaline irritant.

The potassium may be found by incinerating a portion of the substance and testing for it in the ash. Taylor (Med. Jurisprudence) improperly says that the salt itself (cyanide of potassium) may be recovered from the organs by incinerating them in close vessels and treating the ash with water. I have already mentioned that cyanide would be formed in this way from the organic matters themselves, even if not originally present.

B. Mercuric Cyanide, Hg(CN)₂. Of all metals mercury has most affinity for HCN, mercuric oxide decomposing other cyanides, even Prussian blue, and dissolving readily, as we have seen, in free HCN, or in alkaline cyanides. Hence if a compound of mercury have been given medicinally, the prussic acid will be found in the stomach as mercuric cyanide, which is easily soluble in water, neutral to test paper, quite inodorous, and extremely poisonous. It is not officially recognised in any Pharmacopoeia, except the French; has been occasionally used in medicine instead of mercuric chloride, which it resembles in action, but has the advantage of not being incompatible with alkalies and organic matters (Royle’s Mat. Med., 6th ed.). It crystallizes in anhydrous four-sided obliquely-truncated white opaque prisms, with a disagreeable metallic taste, is permanent in the air, easily soluble in water, less in alcohol. It fails to respond to the silver nitrate (partially) or Prussian blue tests, and gives the sulphur test with difficulty. It is decomposed by distillation with hydrochloric acid, but only ?rds of the HCN pass over into the distillate, unless ammonium chloride be added (Roscoe and Schorlemmer’s Chemistry). Whenever HCN is looked for, it is safer to examine also for mercury, and, if found, to add a little hydrochloric acid and sulphuretted hydrogen to the original liquid, thereby precipitating mercuric sulphide (black) and liberating the HCN, which may be distilled off. If, however, excess of sulphuretted hydrogen has been inadvertently added, it would blacken silver nitrate, and hence the silver test would not be available, unless the solution was previously shaken with lead carbonate to remove the sulphide. But it would not affect the Prussian blue or sulphur tests, as sulphide of iron is soluble in hydrochloric acid. Mercuric cyanide also gives off all its HCN when distilled with iron filings or zinc dust, sulphuric acid, and water. This seems a better method.

Mercuric cyanide is said to be an irritant poison, and to be similar in its action to corrosive sublimate. Combination with mercury seems to mask the physiological action of HCN, just as it does its chemical action. The medicinal dose is ¹/_16th grain gradually increased to ½ grain, in pills or solution (Royle). 10 grains have proved fatal. By heat, when dry, it is broken up like silver cyanide into mercury and cyanogen.

C. Cyanides of the Heavy Metals, as zinc, lead, copper, &c. Silver cyanide has already been described. These are insoluble in water, inodorous, and probably, while intact, not poisonous. But they are decomposed by mineral acids, and, as the gastric juice is acid, they would more or less readily yield free HCN, with its usual odour and effects. The influence of the metal has also to be considered.

D. Double Cyanides, derived from iron, cobalt, manganese, chromium, platinum, &c., are inodorous. Those of the alkalies and alkaline earths are alone soluble. The only common ones are ferro-and ferricyanide of potassium, the so-called yellow and red prussiates of potash. They are said to be merely purgative, not poisonous, but, from the comparative facility with which they yield HCN by acids, they cannot be considered safe. Soluble ferrocyanides give, with pure ferrous sulphate, a white precipitate turning blue in air; with ferric chloride a precipitate of Prussian blue; with cupric sulphate a maroon precipitate. Ferricyanide solutions give with ferrous salts a deep blue precipitate; with ferric salts a dark-brown coloration. These reactions would be applied to a filtered portion of the stomach contents. Prussian blue is ferric ferrocyanide mainly, but varies in composition: it is supposed to be inert.

Almen states (Chem. Centr. 1872, 439) that potassium ferrocyanide in solution decomposes at ordinary temperatures, especially if a little free acid be present, HCN being formed. Prussian blue only decomposes when warmed to 40° or 50° C. (therefore not in the body, C. G. S.), “hence the presence of HCN, if accompanied by ferrocyanide, is not a proof of poisoning.” But ferrocyanide is not in any Pharmacopoeia, and is not administered medicinally. Yet, to answer a possible question, a known fraction of the original substance might be extracted with water, and tested as above. The same observations apply to ferricyanide.

When ferro-or ferricyanides are distilled with moderately strong sulphuric acid, a portion of the contained HCN passes over; in fact, this is the common process for preparing prussic acid. The iron remains behind in the retort, in combination with potassium and the rest of the cyanogen. If ferric hydrate —“ferri peroxidum humidum”), or ferrous sulphate and potash, have been administered as antidotes to HCN, Prussian blue might be formed in the stomach. It would then show a blue colour, either by itself or on addition of an acid, and blue particles under the microscope, if in sufficient quantity. In this case the HCN left in the stomach would have been rendered innocuous, and the prussic acid which had actually caused the death would be found free in the blood, &c. The stomach contents might then show no HCN, either by odour or distillation, as Prussian blue is inodorous, and not easily decomposed by dilute acids. With alkalies it turns brown, giving ferric hydrate and an alkaline ferrocyanide.

Ludwig and Maushner (Chem. Centr. 1881, 43), in a case of poisoning, discovered a quantity of potassium ferrocyanide in the body. This was removed by slightly acidulating and carefully precipitating by ferric chloride. The filtrate, distilled with tartaric acid, yielded much HCN. The sample of cyanide of potassium, which had probably caused death, was afterwards found to contain a large proportion of ferrocyanide.

E. Sulphoncyanides (Thiocyanates). Those of the alkalies and alkaline earths are soluble and colourless; ferric sulphocyanide is soluble, and intense blood-red (sulphur test); other sulphocyanides are mostly insoluble. They are all inodorous, poisonous in moderate quantities, and are not officinal in any Pharmacopoeia. Distilled with acids they break up, HCN being found in the distillate. It has been mentioned already that ammonium sulphide, produced by putrefaction, may combine with any HCN present to form ammonium sulphocyanide; therefore, if the matters to be examined are alkaline, and putrefaction has commenced, Allen (Commerc. Org. Anal., 1879, art. HCN) recommends us to digest with alcohol, filter, evaporate to dryness on a water bath, redissolve in a little water, filter again, and test the filtrate with ferric chloride after just acidulating with hydrochloric acid: the well-known blood-red colour will result (see “Sulphur Test”). But the ordinary distillation with tartaric or sulphuric acid would in this case also detect the HCN, though the whole might not pass into the distillate.

Sulphocyanide of mercury is the toy called “Pharaoh’s Serpent.” A case of poisoning by it is recorded.

It is important to notice that traces of sulphocyanide are naturally present in the saliva. If this salt be found, the question will occur, how much could be accounted for by the saliva? Carpenter (Princ. of Human Physiol.) quotes Harley to the effect that the average daily secretion from the salivary glands is 1 or 2 pounds: other observers have stated that it varies greatly. The secretion itself is said to contain, in 1000 parts, one part (Frehrichs), or 0·6 part (Jacubowitsch), or even 0·3 part (Oehl), of potassium sulphocyanide; that is, 4·2 to 7 grains per pound, equivalent to from 1 to 2 grains of HCN, or 2 to 4 grains if 2 pounds of saliva were secreted. This would be a serious matter but for the fact that, whether from decomposition by the gastric juice or otherwise, or from its passing out of the stomach as it passes in, it is certain that no such quantity is ever found naturally in the stomach, not more than a minute trace being ever given by the processes, unless hydrocyanic acid, in one of its forms, has actually been administered.

Cyanide of cadmium, and some of its double salts, are sparingly soluble. Double cyanide of silver and potassium is soluble and crystallizable. It is the salt used in electro-plating, and, as commonly met with, smells strongly of potassium cyanide. Zinc-potassium cyanide has been used medicinally: it occurs in beautiful crystals, inodorous when dry, but having a faint odour of HCN in solution.

The other cyanides are rare, and their physiological action is unrecorded. Cyanic acid and cyanates are said not to be poisonous.

Oil of Bitter Almonds.—The crude oil contains, as we have seen, 8 to 15 per cent, of HCN. Dissolved in spirit it forms “essence of almonds,” and is exceedingly poisonous, having caused thirty-one deaths in four years (Taylor). Two drachms of the oil has killed a man in seventeen minutes (Lancet, 1868, p. 447), two ounces caused death immediately. The odour of almonds is always distinct in the stomach.

The oil can be freed from HCN, but then does not keep so well, and is much more costly. Its sp. gr. is 1·049; it boils at 356° F. The crude oil is yellow: with concentrated sulphuric acid it gives a crimson-red colour, and on diluting a yellow emulsion. We may estimate the amount of HCN in it by shaking with water, separating, adding dilute potash to the aqueous liquid, and testing it with standard silver solution as described under “Silver Test.” The other tests may also be used to prove the presence of HCN; the guaiacum and copper paper being specially convenient.

A case of poisoning by bitter almonds is reported in the “South Australian Register” for August 6th, 1879. A female child (whose age is not stated) ate a dozen of them, freshly taken from the tree, and died in three hours. The symptoms described are pain, coma, and convulsions.

Antidotes to HCN are generally useless since the death is so sudden. A moderately dilute solution of an alkali, such as potash, lime or washing soda, along with a little ferrous sulphate, would render harmless so much of the poison as was still in the stomach unabsorbed. As already mentioned, this would cause a little difficulty in the chemical analysis. Ammonia acts as an antidote by opposing the depressant action of HCN. Chlorine water has been used: this converts the HCN into ammonium chloride, carbon monoxide and dioxide, and a little cyanogen chloride.

Medicinal uses.—Its primary action is on the cerebrospinal nerves. It is employed externally, largely diluted, to allay neuralgia and itching of the skin, and to relieve earache (not more than two drops of B. P. acid at a time)[24]: it must not come in contact with abrasions, or it might be absorbed and produce poisoning symptoms. Internally, it allays dyspepsia and the irritant effects of capsicum, &c. (Royle). Safe dose internally two to six minims of the B. P. 2 per cent. acid, suspended if there is any constriction of the throat (Farquharson’s Therapeutics).

Fatal dose.—Smallest recorded (Med. Gaz. 35, p. 896); twenty grains of Scheele’s acid, fatal in twenty minutes, equal to fifty grains of B. P. 2 per cent. acid, equal to one grain of anhydrous prussic acid. Largest dose with recovery (Lancet, 1854, January 14), one drachm (sixty grains) of Scheele’s acid, but in this case energetic remedies were at once applied. Average fatal dose of 2 per cent. acid, thirty minims (Royle’s Mat. Med., Dr. Harley, 6th ed.).

Symptoms.—These vary with the dose, &c. A large quantity kills in two to five minutes, though insensibility may ensue in a few seconds. But patients may survive for twenty minutes, or even for an hour; and may continue in imminent danger for several hours, and yet recover (Guy and Ferrier, Forens. Med., 1881). Many cases have occurred of voluntary acts, such as concealing or throwing away the bottle, having been performed after fatal doses had been swallowed (Ibid, p. 600). In animals, according to Mr. Nunneley, there is usually a peculiar plaintive cry, but not in man, though there may be a call for assistance. Convulsions, and involuntary evacuation of fÆces or urine, may or may not occur. Large doses kill by cardiac syncope; smaller ones by paralysis of the respiratory centre, or, if gradual, by impeded oxidation of the blood (Farquharson’s Therapeutics). Other symptoms are, dilatation of pupils, muscular prostration, deep convulsive breathing at long intervals, quick feeble irregular pulse, spasmodic closure of the jaws and clenching of the hands (Taylor). Breathing sometimes stertorous (Christison, Ed. Month. Journal, February, 1850, p. 97. Reg. v. Burroughs, Cent. Crim. Court, February, 1857). Vomiting occasional, or foaming at the mouth.

Post-mortem appearances.—Not characteristic (Farquharson; Guy and Ferrier). Putrefaction not accelerated (Taylor). The veins contain dark fluid blood: the right side of the heart is gorged (Harley). There may or may not be congestion and reddening of stomach and intestines, or of the brain. On the whole, the appearances are those of asphyxia.[25] The odour should be sought for in all parts, and as soon as possible the organs should be shut up in stoppered jars, or well-corked and sealed bottles, and sent at once for analysis.

The symptoms and post-mortem appearances of poisoning by Cyanide of Potassium are the same as those of prussic acid, except that:—

1. Convulsions are more common.

2. Owing to the irritant action of the alkali, the stomach is reddened.

3. The contents are alkaline.

The fatal dose is less than five grains, but Taylor mentions a case of recovery after nearly one ounce of the commercial cyanide, which may, however, have contained much carbonate.

Hydrocyanic acid is not, in the strict sense, a cumulative poison; “but doses that exceed the proper medicinal limit may happen to prove fatal though similar previous ones have appeared to be harmless, in consequence of a change in the body itself.” (Guy and Ferrier’s Forensic Medicine, 1881, p. 606.)

In the trial of George Ball for poisoning his mother with prussic acid, at Lewes, July, 1860 (previously reported), the question arose as to the difference between minims and drops. A minim of water is supposed to weigh a grain: if the fluid is heavier than water, it weighs more than a grain; if lighter, it weighs less. But a drop is quite an indefinite quantity: it is affected, not only by the specific gravity, but by the cohesion of the fluid, by the shape and size of the vessel, the manner of pouring, and the temperature. I have made some experiments which show the irregularity. (See also Woodman and Tidy’s Forensic Medicine, p. 456.)

Proving that while a drop may be estimated at about 1½ to 2 minims (a good deal more than the usual supposition, the two terms being often regarded as synonymous), yet the inconstancy is so great that it is absolutely imperative, in using powerful medicines, to prescribe exact measurement, and not such a precarious process as dropping.

As to the period after death during which HCN may be detected, Allen (Comm. Org. Anal.) asserts that its detection is rarely possible after more than twenty-four hours. This is astonishing, as Casper separated more than 18 milligrammes from a body eight days after death; Sokoloff detected it in hounds sixty days after; Dragendorff after four weeks in a dog, after eight or ten days in man. Reichardt (Arch. Pharm. 3, 19, 204) found it in a body two months after death—in the organs, but not in the urine. In the Tawell trial, also, the interval was considerable.

Casper states in his Handbook (vol. iii., illustrative cases of HCN) that Schauenstein (one of the Prussian official chemists), twenty-six hours after death, found no HCN in the stomach, but a considerable amount of formic acid, the result of its metamorphosis. We know that strong HCN, exposed to light, decomposes into formate of ammonium, which, by distillation with a dilute acid, would give formic acid in the distillate. That such a change should occur so rapidly in a dilute solution, and in the darkness of the body, is improbable. It would be well, however, that formic acid should be looked for in the distillate thus:—

Carefully neutralize a measured portion with pure soda or potash; evaporate on the water-bath to dryness. The alkaline formate will be left in white crystals if present, together with the cyanide, which will not crystallize, but remain as a deliquescent mass. Dissolve in a little water, and divide into three equal portions.

(1.) To the first add silver nitrate in slight excess. Cyanide of silver will precipitate, formate will remain in solution, if not too concentrated. Filter, if possible. On boiling, if any blackening happens from reduction of the silver, formic acid is probably present. Acetic acid does not reduce silver nitrate.

(2.) To the second add dilute neutral ferric chloride (a solution of iron-alum answers admirably). A red-brown colour, removed by a drop of hydrochloric acid, indicates either acetic or formic acid.

(3.) Evaporate the third portion to dryness, and ignite gently in a closed crucible. Formate and acetate will be turned into carbonate, while cyanide will remain unchanged if air be excluded. If then effervescence take place on treating the residue with a little hydrochloric acid, it is a confirmation of the presence of formic or acetic acid. The first test will have revealed which it is.

Many animal substances, when distilled with strong acids, do give acetic and formic acids, but they do not act thus with dilute acids. Yet a stomach will usually yield a little acetic acid from the food having turned sour.

If formic acid be present, it will probably have proceeded from the decomposition of HCN. Then the reduced silver obtained in the first test should be weighed, and calculated into formic acid, and also into hydrocyanic acid (108 parts of silver = 46 parts formic acid, or 27 parts HCN). The result may be stated thus:—

“Hydrocyanic acid actually found,—— grains. Formic acid found,—— grains. If this had proceeded from the decomposition of hydrocyanic acid, it would correspond to an additional amount of—— grains of hydrocyanic acid.”

It is needless to observe that the mere finding of formic acid would be no proof of the administration of HCN, unless strong corroborative evidence were at hand.

On the whole, we must always try, and we may often hope, to find HCN if given, either free, as cyanide, or as sulphocyanide, even after months have elapsed.