able time in a phial without any tendency to decomposition Class VI. being perceived. The deutoxide of manganese absorbs completely the vapour of hydrocyanic acid in a few hours. cyanogen is not evolved, Water is formed, but Sect. II. of mercury. acid a violent When red oxide of mercury is heated in hydrocyanic acid of red oxide vapour a violent action takes place, and so much heat is evolved as to destroy the compound formed. When the experiment is made without the application of heat, the vapour is absorbed by the oxide. When the oxide thus saturated with vapour of hydrocyanic acid is heated, water is disengaged, and there remains the substance formerly known by the name of prussiate of mercury. It is obvious from this that a double decomposition takes place, water is formed and cyanodide of mercury. Hydrocyanic acid is one of the most virulent poisons known, Hydrocyanic Some years ago, a chemist, while engaged in preparing it, let a poison. few drops fall upon his arm. It speedily evaporated, yet the unfortunate man fell a victim to the application in the course of two hours. When a single drop of it is applied to the eye of a common sized dog or a cat violent convulsions are produced, which speedily terminate in death. It has been long known that ammonia constitutes an antidote to this poison by counteracting its effects. But from the experiments of M. Simeon,+ Antidote. confirmed by those of MM. Persoz and Norrat,‡ it appears that chlorine constitutes a more certain and complete antidote. Being put into the throat of the poisoned animal it speedily relieves the symptoms, and the animal, after an interval of some hours, again recovers its health. Even in one case where the poison had been allowed to exert its action till the animal was at the point of death, the respiration having been stopped for 25 seconds, the application of chlorine restored animation, and the animal recovered. cine. Hydrocyanic acid when much diluted with water is also a Used in medi poison when administered in sufficient quantity. But when given in small quantities it has been found a valuable medicine in various diseases, and in none does its administration appear to be more beneficial than in dyspepsia. I have found that water holding in solution one per cent. of this acid constitutes a sufficient strength. Of this, as much as a scruple or even half a drachm may be given twice a day. But I have found that about 8 drops of it administered in a glass of water consti * Berzelius; Traité de Chimie, ii. 211. Ibid. xliii. 324. Chap. I. How de. tected. tutes a sufficient dose. When given in larger doses it diminishes the velocity of the pulse to a very remarkable degree. On one occasion I reduced the pulse of an individual to 23 beats per minute. Upon withdrawing the medicine the pulse gradually resumes its natural velocity. If the use of the medicine be persisted in after the pulse has been thus diminished, the patient is thrown into a state of lethargy similar to that so well described by Shakespeare in the tragedy of Romeo and Juliet. A lethargy of this kind, into which a lady was thrown in Glasgow, continued about 30 hours, and she recovered again without any bad consequences whatever following. I mention these facts to show with what caution so powerful a medicine should be prescribed. Lassaigne has proposed the following method of detecting hydrocyanic acid after death in the stomachs of those who have been poisoned by that baneful drug: The stomach is to be cut in pieces and mixed (together with its contents) with a little water. This mixture is to be distilled by a gentle heat till about one-eighth part of the water has passed over into the receiver. To the distilled liquid add a drop of caustic potash, and immediately after a very small quantity of the solution of sulphate of copper. A small quantity of matter will be disengaged by the action of the alkali on the copper solution. Then add one or two drops of muriatic acid. If no hydrocyanic acid be present the whole cupreous precipitate will be dissolved, and the liquid become transparent. But if any hydrocyanate of copper be present it will remain undissolved and of a white colour. By this method hath part of the liquid of hydrocyanic acid may be detected. There is, however, a source of ambiguity. The same white precipitate will remain if the liquid should contain hydriodic acid. Sulphate of iron when substituted for sulphate of copper will detect 5th of the weight of the liquid of hydrocyanic acid, but it has the advantage of being characteristic in consequence of the forma tion of prussian blue.* If the quantity of hydrocyanic acid present be very small the hydrocyanate of copper is redissolved in the course of a few hours. 100 The same difficulty occurs in determining whether hydrocyanic acid unites with bases, as was mentioned before respecting muriatic acid. Yet I think that the phenomena leave little doubt that no such class of salts as hydrocyanates exist. * Ann. de Chim. et de Phys. xxvii. 200. But that whenever this acid comes in contact with an oxide a double decomposition takes place, water being formed while the cyanogen and the metallic base uniting constitute a cyanodide. SECTION III.-OF CYANOUS ACID. Class VI. Sect. III. I have been induced to give this name to the acid first dis- History. covered and described by M. Wöhler, under the name of cyanic acid, in consequence of the subsequent discovery by Serullas of another compound of cyanogen and oxygen, which contains twice as much oxygen as Wöhler's acid.+ To this last acid we must, according to the rules of nomenclature, apply the name cyanic acid. The existence of cyanous acid was first suspected by Vauquelin. He observed that when water impregnated with cyanogen was set aside it underwent spontaneous decomposition, being converted into carbonic acid, hydrocyanic acid, charry matter, ammonia, and a particular acid which he proposed to distinguish by the name of cyanic acid.‡ Wöhler found that when a current of cyanogen gas was passed into barytes water containing undissolved crystals of hydrate of barytes, the liquid assumed a brown colour, and charry matter was deposited while the barytes dissolved. After the process was over, he passed a current of carbonic acid gas through the liquid to decompose any hydrocyanate of barytes that it might contain. It was then boiled to drive off the hydrocyanic acid and concentrated after filtration. A white salt was obtained in silky needles, which was cyanite of barytes.§ In his paper on this subject, he examined the properties of this cyanite and of several others which he obtained; but he did not succeed in procuring the cyanous acid or in analyzing it. But finding that water and his acid by mutual decomposition were converted into carbonic acid and ammonia, he concluded that his acid was a compound of Chap. I. Formation. Properties. It is evident from this that one atom of cyanous acid and three atoms of water may be converted into an integrant particle of bicarbonate of ammonia. Wöhler found afterwards that the easiest method of procuring cyanite of potash was to reduce anhydrous ferroprussiate of potash to a fine powder, and mix it intimately with about its own weight of black oxide of manganese. This mixture is to be heated in a silver crucible to incipient ignition.* The matter which has been thus heated is to be boiled in alcohol of the specific gravity 0-832. The alcoholic solution as it cools. deposites cyanite of potash in small scales, similar in appearance to chlorate of potash. He now subjected the acid to analysis, and confirmed his former opinion of its constitution. Wöhler's attempts to obtain this acid in a separate state were unsuccessful; but Liebig pointed out a process by which it might be procured nearly pure, and so that its characters could be recognised. His analysis at first differed from that of Wöhler, but he afterwards satisfied himself that Wöhler's determination was correct. Cyanous acid may be obtained by the following process: Mix together solutions of cyanite of potash and nitrate of silver. The cyanite of silver precipitates in the state of a white powder, which is nearly insoluble in water. Suspend this cyanite in water, and pass through the liquid a current of sulphuretted hydrogen gas, taking care not to add enough to decompose the whole cyanite of silver employed. The silver is thrown down in the state of sulphuret, and the cyanous acid dissolves in the water. Let the sulphuret of silver and undecomposed cyanite subside, and then decant off the clear liquid, which constitutes an aqueous solution of cyanous acid. taste, and reddens vegetable blues. Wöhler to that of acetic acid. This liquid has a sour Its smell is compared by When left in contact with water it undergoes spontaneous decomposition in a few hours, being converted into carbonate of ammonia.§ It combines * Too great a heat destroys the acid. Ann. de Chim, et de Phys. xxvii. 196; and Gilbert's Annalen, lxxiii. 157. Liebig, Ann. de Chim. et de Phys. xxxiii. 20s, Sect. IV. with the different bases, and neutralizes them, constituting Class VI. cyanites, some of which have been examined, and will be described in a subsequent part of this work. * To determine the atomic weight of this acid, Wöhler analyzed cyanite of potash and cyanite of silver. He found them composed as follows: The mean of the two numbers attached to cyanous acid is 4.35, which therefore is the atomic weight of this acid as derived from the experiments of Wöhler. To determine the composition of this acid he decomposed a given weight of cyanite of silver by means of muriatic acid, which converts the carbon in the acid into carbonic acid gas. The volume of carbonic acid gas being measured gave him the weight of carbon, which the acid in the salt contained. Cyanogen being a compound of 1.5 by weight of carbon and 1·75 of azote, it was easy to infer the weight of azote in the acid. What was wanted to make up the whole weight of the acid was considered as oxygen. In this way he determined the constituents of cyanous acid to be Atomic weight. Carbon 35.334 So that, according to this analysis, the true atomic weight of cyanous acid is 4-25, and it is a compound of 1 atom cyanogen and 1 atom oxygen. SECTION IV.-OF CYANIC ACID. Composition, This acid was discovered by Serullas in 1829. He obtained History. it by dissolving a substance, which he called perchloride of • Ann. de Chim. et de Phys. xxvii. 196. |