Scientific American 28, 1.4.1848
The following is the plan pursued by Professor Loomis to detect prussic acid in the stomach of Mr. Matthews, murdered at Hallowel, Maine. The most volatile poison is prussic acid—therefore it was searched for first. He strained the substance through a linen cloth, leaving the solid parts in the cloth The fluid was placed in a retort, and heated, and the vapor condensed. Previously a small part of the fluid was taken on a piece of paper; and a drop of the solution of pure potassium, a drop of the solution of sulphate of iron and a drop of sulphuric acid put on the paper—this gave a blue color as far as it spread. It indicated the presence of prussic acid, though not with positive certainty. He then took the distilled portion and divided it into three parts. To one portion he added a small quantify of potassium, then a solution of iron, and a drop of sulphuric or muriatic acid. The potassium produced no effect—the sulphate of iron showed a turbid yellow—the acid showed a deep blue color. This indicated prussic acid. The second portion was tested with potassium, sulphate of copper—then with muriatic acid. The effect of the potassium of sulphate of copper was mach as before; but when the acid was applied it produced a white color partially clouded, which soon subsided. This too, indicated prussic acid. The third portion was tested with nitrate of silver—it gave a white curdled precipitate. This white precipitate would be produced by prussic acid and by several other substances—but nothing but prussic acid would produce the curdy appearance. The precipitate was dried, heated, and a lamp applied to the retort. If there had been pure acid sufficient to fill the retort with cyanogen, it would have produced a peach-colored flame which in this case was not obtained. This experiment was repeated on the following Monday, and, during the intervening time, the retort was carefully corked. Then witness washed the solid portion left in the cloth. The washing having been added to the liquid before in the retort, from the whole there was now distilled nearly an ounce of transparent liquid. This was treated with nitrate of silver, which produced the curdy precipitate before described. This precipitate was dried, and placed in a glass tube an inch and a half in length, sealed at one end, and drawn out to a capillary tube at the other. On heating the precipitate, thus enclosed, cyanogen escaped from the capillary extremity, which instantly ignited, producing a distinct peach-blow flame. This flame is produced only by cyanogen gas, which is the base of Prussic acid. These tests are the ordinary and approved tests of Prussic acid. The first test applied was sulphate of iron. Hydrocianic acid is a compound substance. The substance that produces the blue color is cyanide of iron. Cyanogen is derived from Prussic acid. There is no other combination of the elements present that will give this color. Cannot say how long the tests now used for the discovery of narcotic poisons have been employed—know that there is no other combination of iron that will produce this color as well as he knows any other principle in science. The odor of Prussic acid owes its peculiarity to neither of the elements independently but to the elements in their compound state. There is an odor to cyanogen —witness had experiments with it. It is always gas. The silver test produces a curdy precipitate which must be a compound of silver. Pure cyanogen will produce the peach blow flame. It will combine with other substances.
The above experiments will be read with interest, as they contain important information relative to chemical analysis.