and, when cold, add a little of a concentrated solution of sulphurous acid in water; warm until the excess of the sulphurous acid is driven off, and then conduct hydrosulphuric acid into the fluid. If arsenic is present, a yellow precipitate will form. When the precipitate has completely subsided, and the fluid has nearly lost the smell of sulphuretted hydrogen, filter, wash the precipitate, dissolve it in ammonia, and proceed with the solution as directed in 5, (297), to determine the weight of the arsenic.

7. Reduction of the Sulphide of Arsenic.

The production of metallic arsenic from the sulphide, which may 300 be regarded as the keystone of the whole process, demands the greatest care and attention. The method recommended in § 131, 12, viz., to fuse the arsenical compound, mixed with cyanide of potassium and carbonate of soda, in a slow stream of carbonic acid gas, is the best and safest, affording, besides the advantage of great accuracy, also a positive guarantee against the chance of confounding the arsenic with some other body, more particularly antimony; on which account it is more especially adapted for medicolegal investigations.

As regards the process of reduction, either proceed at once with the sulphide of arsenic, or previously convert the latter into arsenic acid (see 301). In the former case take care, if possible, not to use the whole of the residue in the dish, obtained by the evaporation of the ammoniacal solution, but only a portion of it, so that the process may be repeated several times, if necessary. Should the residue be too trifling to admit of being divided into several portions, dissolve it in a few drops of ammonia, add a little carbonate of soda, and evaporate on the water-bath to dryness, taking care to stir the mixture during the process; divide the dry mass into several portions, and proceed to reduction.

Otto* recommends to convert the sulphide first into arsenic acid, 301 and then to reduce the latter with cyanide of potassium. The following is the process given by him to effect the conversion of the sulphide into the acid; pour concentrated nitric acid over the sulphide of arsenic in the dish, evaporate, and repeat the same operation several times, if necessary, and then remove every trace of nitric acid by repeatedly moistening the residue with water, and drying again; when the nitric acid is completely expelled, treat the residue with a few drops of water, add carbonate of soda in powder, to form an alkaline mass, and thoroughly dry this in the dish, with frequent stirring, taking care to collect the mass within the least possible space in the middle of the dish. The dry mass thus obtained is admirably adapted for reduction. I can, from the results of my own experience, fully confirm this statement of Otto; but I must once more repeat, that it is indispensable for the success of the operation that the residue should be perfectly free from every trace of nitric acid or nitrate; otherwise deflagration is sure to take place during the process of fusion with cyanide of potassium, and, of course, the experiment will fail.

*

“Anleitung zur Ausmittelung der Gifte,” von Dr. Fr. Jul. Otto, p. 36.

When the operation is finished, cut off the reduction tube at c (see 302 Fig. 30), set aside the fore part, which contains the arsenical mirror, put the other part of the tube into a cylinder, pour water over it, and let it stand some time; then filter the solution obtained, add to the

Fig. 30.

h

filtrate hydrochloric acid to acid reaction; then again some hydrosulphuric acid, and observe whether this produces a precipitate. In cases where the reduction of the sulphide of arsenic has been effected in the direct way, without previous conversion to arsenic acid, a trifling yellow precipitate will usually form; had traces of antimony been present, the precipitate would be orange-colored and insoluble in carbonate of ammonia. When all the soluble salts of the fused mass have been dissolved out, examine the metallic residue, which may be left behind, according to the directions of § 133, 1, for traces of tin and antimony; these being the only metals that can possibly be present if the instructions here given have been strictly followed. Should appreciable traces of these metals, or of either of them, be found, proper deduction and correction must be made in calculating the weight of the arsenic.

8. Examination of the reserved Residues, marked severally I., II.,
III., and IV., for other Metals of the Fifth and Sixth Groups.
a. Residue I. Compare § 223, 1 (290).

i

This may contain chloride of silver and sulphate of lead, 303 possibly also binoxide of tin. Incinerate the residue (I.) in a porcelain dish, burn the carbon with the aid of some nitrate of ammonia, extract the residue with water, dry the part left undissolved, and then fuse it with cyanide of potassium in a porcelain crucible. When the fused mass is cold, treat it with water until all that is soluble in it is completely removed; warm the residue with nitric acid, and proceed as directed in $179.

b. Residue II. Compare § 223, 3 (292).

The carbonaceous residue which is obtained by the purifica- 304 tion of the crude sulphide by means of nitric acid and sulphuric acid, may more particularly contain lead, mercury, and tin; antimony and bismuth may also be present.

Heat the residue for some time with nitrohydrochloric acid, and filter the solution; wash the undissolved residue with water mixed with some hydrochloric acid, add the washings to the filtrate, and treat the dilute fluid thus obtained with hydrochloric acid; should a precipitate form, examine this according to the instructions given in § 189. Incinerate the residue insoluble in nitrohydrochloric acid, fuse the ash in conjunction with cyanide of potassium, and proceed with the fused mass as directed in 8, a (303).

c. Residue III. Compare § 223, 6 (298).

Examine the precipitate insoluble in sulphide of ammonium 305 for the metals of the fifth group according to the instructions given in § 191.

d. Residue IV. Compare § 223, 6 (299).

This may contain tin and antimony, perhaps also copper. Proceed as directed § 190, 2, b (123). If the color of the residue was black (oxide of copper), treat the reduced metals according to the instructions given in § 179.

9. Examination of the Filtrate reserved in § 233, 2 (291), for Metals of the Fourth and Third Groups, especially for Zinc and Chromium.

306

a. As we have seen in § 223, 2 (291), the fluid filtered from 307 the precipitate produced by hydrosulphuric acid, and temporarily reserved for further examination, has already been mixed with sulphide of ammonium. The addition of this reagent to the filtrate is usually attended with the formation of a precipitate, consisting of sulphide of iron and phosphate of lime, but which may possibly also contain sulphide of zinc. Filter the fluid from this precipitate, and treat the filtrate as directed in b (308); wash the precipitate with water mixed with some sulphide of ammonium, dissolve by warming with hydrochloric acid, and boil the solution with nitric acid, to convert the protoxide of iron into sesquioxide; add, if necessary, sufficient sesquichloride of iron for carbonate of soda to produce a brownish-yellow precipitate in a sample of the fluid; neutralize almost completely with carbonate of soda, precipitate with carbonate of baryta, and filter; the precipitate contains all the sesquioxide of iron and all the phosphoric acid. Concentrate the filtrate, precipitate the baryta with dilute sulphuric acid, filter, add to the filtrate ammonia to alkaline reaction, and precipitate with sulphide of ammonium the zinc which may be present. For the further examination of the precipitate, see § 105.

b. If the analyzed substance contained chromium, this will 308 be found in the fluid filtered from the precipitate produced by sulphide of ammonium, § 223, 2 (291). Compare § 223, 9, a (307). If you wish to ascertain whether chromium is really present, evaporate the filtrate to dryness, mix the residue with 3 parts of nitrate of potassa and 1 part of carbonate of soda, put the mixture into a Hessian crucible and heat to moderate redness. Allow the fused mass to cool, and, when cold, boil with water yellow coloration of the fluid shows the presence of alkaline chromate, and accordingly of chromium. For confirmatory tests, see § 138.

II. METHOD FOR THE DETECTION OF HYDROCYANIC ACID.

$224.

In cases of actual or suspected poisoning with hydrocyanic acid, 309

where it is required to separate that acid from articles of food or from the contents of the stomach, and thus to prove its presence, it is highly necessary to act with the greatest expedition, as the hydrocyanic acid speedily undergoes decomposition. Still this decomposition is not quite so rapid as is generally supposed, and indeed it requires some time before the complete decomposition of the whole of the acid present is effected.*

Although hydrocyanic acid betrays its presence, even in minute quantities, by its peculiar odor, still this sign must never be looked upon as conclusive. On the contrary, to adduce positive proof of the presence of the acid, it is always indispensable to separate it, and to convert it into certain known compounds.

The method of accomplishing this is based upon distillation of the acidified mass, and examination of the distillate for hydrocyanic acid. Now, as the non-poisonous salts, ferro- and ferricyanide of potassium, on distillation, likewise yield a distillate containing hydrocyanic acid, it is, of course, indispensable-as Otto very properly observes-first to ascertain whether one of these salts may not be present. For this purpose, stir a small portion of the mass to be examined with water, filter, acidify the filtrate with hydrochloric acid, and test a sample of it with sesquichloride of iron, another with sulphate of protoxide of iron. If no blue precipitate forms in either, soluble ferro- and ferricyanides are not present, and you may safely proceed as follows:

Test, in the first place, the reaction of the mass under examina- 310 tion; if necessary, after mixing and stirring it with water. If it is not already strongly acid, add solution of tartaric acid until the fluid strongly reddens litmus paper; introduce the mixture into a retort, and place the body of the retort, with the neck pointing upwards, in an iron or copper vessel, but so that it does not touch the bottom, which should, moreover, by way of precaution, be covered with a cloth; fill the vessel with a solution of chloride of calcium, and apply heat, so as to cause gentle ebullition of the contents of the retort. Conduct the vapors passing over, with the aid of a tight-fitting tube, bent at a very obtuse angle, through a Liebig's condensing apparatus, and receive the distillate in a small, weighed flask. When about half-an-ounce of distillate has passed over, remove the receiver, and replace it by a somewhat larger flask, also previously tared. Weigh the contents of the first receiver, and proceed as follows:

a. Mix one-fourth of the distillate with solution of potassa 311 or soda to strongly alkaline reaction, and then add a small quantity of solution of sulphate of protoxide of iron, mixed with a little sesquichloride of iron.

b. Treat another fourth as directed § 155, 7, to convert the 312 hydrocyanic acid into sulphocyanide of iron. As the distillate might, however, contain acetic acid, do not neglect to add some

* Thus I succeeded in separating a notable quantity of hydrocyanic acid from the stomach of a man who had poisoned himself with that acid in very hot weather, and whose intestines were handed to me full 36 hours after death.-A dog was poisoned with hydrocyanic acid, and the contents of the stomach, mixed with the blood, were left for 24 hours exposed to an intense summer-heat, and then examined the acid was still detected.

hydrochloric acid after the sesquichloride of iron, in order
to neutralize the adverse influence of the acetate of ammonia.
Compare § 155, 7.

c. If the experiments a and b have demonstrated the pre- 313 sence of hydrocyanic acid, and you wish now also to approximately determine its quantity, continue the distillation, until the fluid passing over contains no longer the least trace of hydrocyanic acid; add one-half of the contents of the second receiver to the remaining half of the contents of the first, mix the fluid with nitrate of silver, then with ammonia until it predominates, and finally with nitric acid to strongly acid reaction. Allow the precipitate which forms to subside, filter on a tared filter, dried at 212° F., wash the precipitate, dry it thoroughly at 212° F., and weigh. Ignite the weighed precipitate in a small porcelain crucible, to destroy the cyanide of silver, fuse the residue with carbonate of soda and potassa-to effect the decomposition of the chloride of silver which it may contain-boil the mass with water, filter, acidify the filtrate with nitric acid, and precipitate with nitrate of silver; determine the weight of the chloride of silver which may precipitate, and deduct the amount found from the total weight of the chloride and cyanide of silver: the difference gives the quantity of the latter; by multiplying the quantity found of the cyanide of silver by 0-2017, you find the corresponding amount of anhydrous hydrocyanic acid; and by multiplying this again by 2-as only one-half of the distillate has been used—you find the total quantity of hydrocyanic acid which was present in the examined mass.

Instead of pursuing this indirect method, you may also deter- 314 mine the quantity of the hydrocyanic acid by the following direct method: Introduce half of the distillate into a retort, together with powdered borax; distil to a small residue, and determine the hydrocyanic acid in the distillate as cyanide of silver. Hydrochloric acid can no longer be present in this distillate, as the soda of the borax retains it in the retort (Wackenroder).

III. METHOD FOR THE DETECTION OF PHOSPHORUS.

§ 225.

Since phosphorus paste has been employed to poison mice, &c., 315 and the poisonous action of lucifer matches has become more extensively known, phosphorus has not unfrequently been resorted to as an agent for committing murder. The chemist is therefore occasionally called upon to examine some article of food, or the contents of a stomach, for this substance. It is obvious that, in cases of the kind, his whole attention must be directed to the separation of the phosphorus in the free state, or to producing such reactions as will enable him to infer the presence of free phosphorus; since the mere finding of phosphorus in form of phosphates would prove nothing, as phosphates invariably form constituents of animal and vegetable bodies.