The results from no single test should be relied upon. As a knowledge of the appearances produced by the four principal re-agents usually employed must greatly facilitate such an examination, we have constructed the following table from actual experiments: comparing the results obtained from solutions of arsenious acid with those from solutions of corrosive sublimate, tartarized antimony, and muriate of baryta, which are the only substances likely to be mistaken for it. It is necessary to remark that the broth employed was made with beef, and contained a moderate proportion of carrots, turnips, and onions, and that the coffee and the tea contained milk and sugar in the usual proportions employed in these beverages.

I shall add here the plan proposed by my late truly excellent friend and colleague, Dr. Turner, and Professor Christison of Edinburgh. They direct the stomach to be cut in pieces and boiled, in a porcelain vessel, in filtered water: then to filter the fluid, and add to it acetic acid sufficient to acidulate it. This acidulated solution is next to be subjected to a stream of sulphuretted hydrogen gas; then boiled, and the precipitate after filtration collected, washed with distilled water, and dried: it is then to be put into the small glass tube (fig. a, page 221), being careful that none of it touch the sides of the tube; to prevent which they should either be guarded, by introducing a roll of paper into the tube, or the small funnel, b, should be used. Next, some black flux must be introduced; and the lower part of the tube cautiously heated over the flame of a spirit-lamp, and next the bulb, until it be raised to full red heat. If arsenious acid was the poison, the decomposition of the sulphuret will afford a ring of metallic arsenic on the tube: but to determine this with more certainty, Dr. Turner proposes to separate the bulb of the tube and any portion of it containing the flux, by melting the tube and drawing it out The crust in the remaining part is next to be driven up and down by the heat of the spirit-lamp, when, if it be arsenic, it will be formed into detached octahedral crystals, white, translucent, and easily recognised under the microscope.

If more proof be wanted, these may be dissolved and tested with ammoniacal nitrate of silver, and ammoniacal sulphate of copper.1 But the certainty and the simplicity of Mr. Marsh's process sets aside almost every other test. In legal inquiries, however, it is always proper to corroborate the results of the several tests by those of all.

1 To ascertain the delicacy of this test, the following experiments were made :- Exp. 1. Halfa drachm of arsenious acid being boiled in 1000 grains of water, and the fluid filtered when cold, it was found to retain twenty-eight grains of the white oxide in solution. Exp. 2. One drachm of this solution being mixed with two ounces of water in a cylindrical glass vessel, so as to form a solution which contained about one part of the acid for 592 parts of water, a scruple of finely-powdered charcoal was added, and the mixture being well agitated with a glass rod, and allowed to settle, was filtered. The powder, when dry, on being thrown upon a red-hot shovel, emitted a very faint odour of garlic. Exp. 3. The same as the former, except that two drachms of the solution were employed making the proportion of the acid to the water in the diluted solution as 1 to 286; the garlic odour was very perceptible. Exp. 4. Four drachms of the arsenical solution being employed, making the proportion of the acid to that of the water in the diluted solution as 1 to about 143, the garlic odour was extremely strong.

From these experiments it is evident that this test will detect arsenic in any solution strong enough to act as a poison.

2 Christison on Poisons, p. 196.

1 Edinburgh Journ, of Med. Science, vol. ii. p. 254.

Comparative Table Of The Precipitates Obtained From Solutions Of Arsenious Acid, Of Bichloride Of Mercury, Of Tartarized Antimony, And Of Muriate Of Baryta, With Different Tests

1 This precipitate, dried upon a filter, and heated with some caustic potassa in a slender test tube, is decomposed in a few seconds, forming a sulphuret of potassa, whilst the arsenic is volatilized in its metallic form, and adheres to the sides of the tube. ( Orfila.)

2 All substances containing tannin in solution greatly impair the solvent influence of fluids in arsenious acid.

3 Mr. Pereira states, that, when the solution of tartar emetic is very dilute, and sulphuretted gas passed through it only for a few seconds, the precipitate is of a lemon yellow, closely resembling that produced by arsenious acid. (Med. Gaz., April, 1836.)

II. Hydro-Sulphuret Of Potassa

1 The hydro-sulphuret, added to a solution of the phosphates, throws down a greenish-yellow precipitate, the supernatant fluid being yellow and turbid. 2 Lime water, also, added to coffee containing arsenious acid, throws down a yellow precipitate; although it precipitates the watery solution of arsenic white. ( Orfila.)

3 Corrosive sublimate cannot be exhibited in port wine with an intention to commit murder (except by a self-murderer), as it changes the colour of the wine to pale violet.

4 All the precipitates by the sulphuret, when dried, and heated in a tube with iron filings, afford metallic mercury, which is volatilised, and shows itself in globules upon the sides of the tube. 5 This test is capable of detecting arsenious acid in a solution containing 1/110000 of its weight. ( Orfila.)

6 It has been suggested, that onions boiled in broth, or eaten so as to impregnate with their peculiar qualities the contents of the stomach, might produce the same effects on ammoniated sulphate of copper, as if arsenious acid were present; but although a green colour is produced, yet no precipitate falls as when arsenic is present.

7 Dr. Porter, of South Carolina, says, that sulp. of copper with ammonia produced the same coloured precipitate in coffee, which contains no arsenic ! American Journ. of Science, vol. iii. p. 354.

8. A similar precipitate is formed by nitrate of silver, in a solution of any of the phosphates; and with chromate of potassa; but the fact of the precipitate being occasioned by arsenic is easily ascertained by testing a fresh portion of the solution with lime-water. If it contain arsenic, a copious white precipitate will be thrown down; if a phosphate only, there is scarcely any change, or at the most a translucent flocculent precipitate, which remains long suspended. A new method of employing this test was suggested by Dr. Paris : it is to put upon a piece of clean white paper a broad streak of the suspected fluid; and then run lightly over it a stick of lunar caustic. This is an excellent test, when modified, as I have elsewhere (London Med. Repository, vol. viii. p. 178.) suggested, by brushing the streak lightly over with liquid ammonia, immediately after the application of the caustic; if arsenic be present, a bright queen's yellow is instantly produced, which remains permanent for nearly an hour; but when the lunar caustic produces a bright yellow before the ammonia is applied, we may suspect the presence of some phosphate rather than arsenic.