A difference of opinion prevails as to the action of the oxide on vegetable colours. This is a matter of no great consequence to the medical jurist; but it is right not to leave a disputed point without some notice. Guibourt says the transparent variety faintly reddens litmus, while the opaque variety faintly restores to blue litmus previously reddened.[1] My own experiments are at variance with these statements: I have always found that the solution of the powder, which is of the opaque variety, faintly reddens litmus, and does not alter reddened litmus.
The remaining chemical properties of the oxide, which it is necessary for the medical jurist to know, will be mentioned under what is now to be said of the principal test by which its presence may be ascertained. Under this head will be noticed, first the tests for the solid oxide, secondly, those for its solution, and lastly, the method of detecting it when mingled with vegetable or animal solids and fluids, such as the contents and tissues of the stomach.
Of the Tests for Arsenic in the solid state.
The most characteristic and simple test for oxide of arsenic in its solid state, either pure or mixed or combined with inorganic substances, is its reduction to the metallic state.
Various methods have been at different times proposed for employing the test of reduction. In the ruder periods of analytic chemistry we find Hahnemann recommending a retort as the fittest instrument, and stating ten grains as the least quantity he could detect.[2] Afterwards Dr. Black substituted a small glass tube, coated with clay and heated in a choffer; and in this way he could discover a single grain.[3] In a paper published in the Edinburgh Medical and Surgical Journal, I showed how to detect a sixteenth of a grain; and afterwards even so minute a quantity as a hundreth part of a grain.[4]
The process is performed in a glass tube; which, when the quantity of the oxide is very small, should not exceed an eighth of an inch in diameter, and may be conveniently used of the form first recommended by Berzelius, and represented in Fig. 3.—The best material for reducing the oxide is recently ignited charcoal, if the quantity of suspected substance be very small. For when any of the ordinary alkaline fluxes is used, more than half of the arsenic is retained, probably in the form of an arseniuret of the alkaline metalloid. But when the quantity of matter for analysis is considerable, charcoal is inconvenient, as it is apt to be projected up the tube on the application of heat; and an alkaline flux is on that account preferable. For this purpose soda-flux,—made by grinding crystals of corbonate of soda with an eighth of their weight of charcoal, and then heating the mixture gradually to redness, so as to drive off all water,—is better than the more familiar black-flux, which con-*
