respectively, of cyanide of potassium,* unless we have previously shown the absence of ferrocyanide of potassium.* Ferrocyanide of potassium, a salt which does not possess poisonous properties, also yields on distillation a liquid which contains hydrocyanic acid, even if the contents of the retort are but very slightly acid, or in presence of even a very weak acid; and this will occur even if the distillation is effected at the lowest possible temperature. Apparently no hydrocyanic acid is given off at first on distilling over a water-bath a mass containing ferrocyanide of potassium; after awhile, however, decomposition takes place, and hydrocyanic acid is evolved. Hence it is necessary to test the substances under examination for the presence of ferrocyanide

*[Of poisonous cyanides the author has only considered the cyanide of potassium, and of not poisonous, the ferrocyanide of potassium. These two compounds may be considered as the types of the two great classes of cyanogen-compounds, the one comprising the poisonous, the other the not poisonous ones.

To the first class-the poisonous cyanogen-compounds-belong all soluble simple cyanides of the constitution, M Cy, and all double cyanides which form no acids. Among the simple cyanides only those of the alkalies and earths, and the cyanides of mercury, and cadmium are soluble in water. Of the insoluble ones, the cyanides of zinc, copper, lead, and silver are under the influence of the living body so decomposed that hydrocyanic acid is set free; they are, therefore, also poisonous. Of the double cyanides of this class, those soluble in water exercise a more powerful poisonous action than those insoluble in water; both kinds are decomposed by dilute acids, especially by hydrochloric acid, and hydrocyanic acid is set free.

The second class-the non-poisonous cyanogen-compounds-comprises all insoluble double-cyanides which show acid properties, and some few of the soluble ones. These are very stable compounds, and decomposable only by powerful acids; they are not acted upon by dilute acids.-Pelikan.]

of potassium before subjecting them to distillation. To this end a portion of the original substance is, if necessary, mixed with water, thrown on a filter, the filtrate acidulated with hydrochloric acid, and a drop of sesquichloride of iron added. The formation of a precipitate of Prussian blue indicates the presence of ferrocyanide of potassium.

§ 83. The question now arises, how to operate when hydrocyanic acid, or cyanide of potassium, and ferrocyanide of potassium occur simultaneously? i. e. how can we show in this case the presence of hydrocyanic acid or cyanide of potassium? To add an excess of carbonate of soda to the mass before subjecting it to distillation, does not lead to a satisfactory result; it is true that the decomposition of the ferrocyanide of potassium is thereby prevented, but the hydrocyanic acid becomes at the same time fixed, so that at the beginning of the operation not a trace of it is obtained, and even at a later period only traces of the acid, resulting from the decomposition of the cyanide of sodium, pass into the receiver. If a solution of carbonate of soda, to which some hydrocyanic acid is added, is heated, carbonic acid escapes, showing that the acid properties of hydrocyanic acid are not so weak as is generally believed.

According to experiments which Mr. Von Pöllnitz made in my laboratory, the best way to follow in this case is this: The mass containing ferrocyanide of potassium, and, eventually, hydrocyanic acid, is mixed with a solution of sesquichloride of iron, when the ferrocyanide of potassium is converted into Prussian blue; carbonate of soda is then added, until the mass exhibits an alkaline reaction, and afterwards tartaric

acid, until it shows a feeble acid reaction; it is then distilled over the water-bath. Ferrocyanide of potassium, when treated in this manner, yields a distillate free from hydrocyanic acid, because the Prussian blue does not suffer decomposition; but if hydrocyanic acid or cyanide of potassium were present, the distillate contains hydrocyanic acid.

[If the presence of ferrocyanide of potassium in the substance under examination is suspected, the mass may be introduced in the retort without addition of an acid, heated to ebullition, then filtered, the filtrate concentrated by evaporation, and mixed with strong heated alcohol; the ferrocyanide of potassium is precipitated in yellowish-white lustrous scales. If cyanide of potassium was present, it remains dissolved in the alcohol; and if the mass contained free hydrocyanic acid, it will be found in the distillate.]

[§ 84. If, after the methods just described, no hydrocyanic acid has been detected in the vomited matters, the stomach and the intestines, other organs and secretions may be subjected to the same treatment, so ex. gr. the blood, brains, liver, and urine, though it will probably be fruitless. In the brain, the spinal marrow, and the heart, hydrocyanic acid has never yet been detected by reagents, though its presence was clearly indicated by the odor; here, therefore, the odor was evidently a more delicate test than any chemical reagent. In the liver and urine, Orfila could never detect this poison.]

[§ 85. To prove satisfactorily the presence of hydrocyanic acid in a dead body is always, unless the circumstances be very favorable, a pretty difficult mat

ter. The chances of obtaining a decisive result decrease in proportion to the time which elapses between the administration of the poison and the dissection of the body. It would appear from repeated experiments that, under ordinary circumstances, the chemical examination is not likely to be attended with success when made three days after death, or eight days after death in bodies that have been buried; this, however, should not detain the analyst to undertake an examination at even a later period, since there are a few cases on record where the poison has been found seven, and even twelve, days after death.

A chemical examination of the body is useless, and cannot possibly yield any result if death was produced by inhalation of hydrocyanic acid vapor, or if chlorine or a hypochlorite (ex. gr. bleaching liquid) has been administered as antidote.-Schneider.]

[§ 86. From the preceding remarks it is evident that the analyst can never be expected to determine, with the slightest degree of accuracy, the quantity of hydrocyanic in a body; to show its presence is all he can do. Even if the analytical methods for its determination were sufficiently accurate to separate and determine the whole amount of the poison contained in the various organs, the blood, the secretions, etc., a considerable quantity would escape detection on account of its volatility. But there are other cases in which a quantitative determination may be desired. Thus ex. gr. the analyst may be required to examine rests of food and drink for hydrocyanic acid, and to determine whether the

quantity of the poison which they contain is sufficient to have caused death. This question can only be answered by a quantitative analysis, which is conveniently executed in the following manner :

The substance under examination, if a liquid, is accurately weighed, then precipitated with nitrate of silver, afterwards a few drops of nitric acid added, just sufficient to produce a weak acid reaction, and the precipitate collected on a filter; the precipitate is washed with distilled water, dried at 212° and weighed; 100 parts of cyanide of silver correspond to 20.15 parts of hydrocyanic acid. If the liquid contains chlorides, besides cyanides, the precipitate is a mixture of cyanide and chloride of silver. In such a case the liquid has to be divided in two equal parts. With one of them we proceed as just mentioned, and thus obtain the sum of chloride and cyanide. The other is mixed with some borax, evaporated to dryness, and the dry residue exposed to a moderate heat (the heat ought never be driven so high as to cause fusion); the hydrocyanic acid vaporizes, while hydrochloric acid unites with the soda of the borax. The residue is dissolved in water, the solution acidulated with nitric acid, and afterwards precipitated with nitrate of silver; the precipitate is dried and weighed. By deducting the weight of the chloride of silver from the weight of the chloride and cyanide, as obtained by the first experiment, we obtain the weight of the latter.

Solid substances are treated as directed (§ 74), care being taken to connect the retort with a good refrige