[249]. See a letter from Mr. Hume on the subject, to the Editors of the Medical and Physical Journal. July, 1810.
[250]. On the detection of very minute quantities of Arsenic and Mercury. By James Smithson, Esq. F.R.S. Annals of Philosophy, August, 1822.
[251]. If any trifling opacity occur in a simple solution of arsenic, when assayed by the nitrate of silver, it may be considered as the effects of some casual impurity; this may be farther demonstrated by bringing over the surface of the arsenical liquid, a piece of blotting paper, or a stopper moistened with a solution of ammonia, when there will instantly form a copious yellow precipitate of arsenite of silver. If this experiment be performed by spreading the mixed solutions of arsenious acid and nitrate of silver over a surface of glass, laid upon white paper, the result will be most striking and beautiful, for on slowly bringing the ammoniacal test over it, the yellow cloud will gradually diffuse itself over the surface.
[252]. Pharmacologia. Edit. 5, vol. ii, p. 96.
[253]. London Medical and Physical Journal, January, 1818.
[254]. The following is the formula for its preparation. Dissolve ten grains of lunar caustic, in ten times its weight of distilled water; to this add, guttatim, liquid ammonia, until a precipitate is formed; continue cautiously to add the ammonia, repeatedly agitating the mixture until the precipitate is nearly redissolved. The object of allowing a small portion to remain undissolved is, to guard against an excess of ammonia. Wherever the test is used, the liquid to which it is added ought to be quite cold.
[255]. This is very important, for an excess of ammonia redissolves the yellow precipitate, and therefore defeats the object of the test. The fixed alkalies, in excess, have not such a property.
[256]. The great impression made upon the public mind in Cornwall by the above trial, produced a disposition to regard every sudden death with more than usual jealousy. In consequence, therefore, of a report having arisen that a young woman had died after an illness of forty-eight hours, and been hastily buried at Madron, near Penzance, the magistrates of that district issued their warrant for the disinterment of the body, and requested the author’s attendance at the examination. The dissection was accordingly conducted in the church, when it appeared that the immediate cause of death had been an inflammation of the intestines; the stomach was found to contain a considerable portion of liquid, which was carefully collected and examined; no solid matter could be discovered in it, nor were any particles found to be adhering to the coats of the stomach. The fluid appeared to consist principally of the remains of a quantity of pennyroyal tea, which had been the last thing administered to the deceased. This was divided into several distinct portions, and placed in separate wine glasses, and submitted, in the presence of the High Sheriff, and some other gentlemen whose curiosity had been excited by the late trial of Donnall, to a series of experiments, amongst which the following may be particularized, as bearing upon the present question, and as affording an important elucidation of it.
A few drops of a solution of sub-carbonate of potass were added to the liquid, in one of the glasses, when its colour, which was originally of a light hazel, was instantly deepened into a reddish yellow; the sulphate of copper was then applied, when a precipitate fell down, which every one present simultaneously pronounced to be of a “vivid grass green” hue; but, on pouring off the supernatant liquid, and transferring the precipitate upon a sheet of white paper, it assumed the blue colour which is so characteristic of the carbonate of copper. The explanation of the phenomenon, and the fallacy to which it gave rise, became obvious; the yellow colour imparted to the liquid by the alkali, was the effect of the latter body upon the vegetable extractive matter of the infusion. The other portions were then strictly examined, but no indications of arsenic or any other metallic poison were discovered.
[257]. This explanation applies equally to the objection lately advanced by Dr. Porter, of the University of South Carolina, who, in his observations on the tests of arsenic, remarks, that an appearance similar to “Scheele’s Green,” is produced by the carbonate of potass, when added to a solution of the sulphate of copper in coffee, but without arsenic, more striking than if even a weak solution of arsenic were used. Silliman’s Journal, iii. 865.