The best fluxes to use are ferrocyanide of potassium dried at 212° F., calcined bitartrate of potassium, cyanide of potassium, and powdered charcoal.
The metallic ring is proved to be arsenical by the properties and tests previously noticed. Should it be imperfectly formed, or masked by decomposed organic matter, the portion of the tube which contains it may be cut off with a file, next coarsely powdered, then reintroduced into another arsenic tube, and the exposure to heat repeated.
The characteristics most simple and well-marked are—
The volatility of the deposit when heated, shown by its escaping from the hotter portion of the tube and condensing on the cooler part higher up or further on.
Its conversion into minute octahedral crystals of arsenious anhydride, when repeatedly chased up and down the tube by the cautious application of the flame of a spirit lamp first to one part, and then to another. The character of these crystals with respect to volatility, lustre, transparency, and form, is so exceedingly well marked that a practised eye may safely identify them, though their weight should not exceed the 1⁄100th or even the 1⁄250th part of a grain. A pocket lens is here serviceable. The form of the crystals is very evident with a microscope of 4 powers. Oxide of antimony never forms octahedrons, but only prisms.
In employing this test, particular care must be taken to avoid soiling the sides of the tube in inserting the mixture, and that the substances operated on are perfectly dry; as unless this is attended to, the experiment does not succeed. The common plan is to introduce the mixture through a small paper funnel or tube extemporised for the purpose. The heat at first should be gentle, and merely sufficient to expel any adhering moisture from the mixture and the inner surface of the tube; after which (except where otherwise ordered) the upper portion of the mixture should be strongly heated, and then the bulb or bottom of the tube exposed to the full flame. After the operation is complete the bulb or lower portion of the tube is usually removed by a file, and the portion containing the deposit hermetically sealed, when it may be preserved, unaltered, for any length of time, ready to be produced as evidence if required.
This test is usually regarded as decisive; as we here actually obtain the arsenic in a solid form, recognisable by the most unequivocal characters.
Reinsch’s Test; Cupro-arsenical Test. The suspected solution is strongly acidulated with hydrochloric acid (1 to 6 or 8), and after being raised to ebullition in a porcelain or glass vessel, a piece of bright and clean metallic copper about 1⁄2 inch long and 1⁄4 inch wide in the form of gauze or foil, but preferably the first, is added, and the whole boiled together. The time required for the ebullition varies according to the strength of the solution; when weak it should be continued for at least a quarter of an hour. When the quantity of arsenic in the suspected liquid is very small, at least half an hour should elapse before the removal of the copper. In solutions containing a notable quantity of arsenic, a few seconds is often sufficient to obtain a coating; but which, for safety sake, may be extended to two or three minutes, or even longer. Liquids rich in organic matter also require longer boiling than those nearly free from it. The coated copper, which has now acquired a characteristic iron-grey colour, is then taken from the liquid, carefully washed in distilled water, in alcohol, and (if greasy) in ether, next dried on blotting-paper, and then either cut into small pieces, or rolled into a small coil or cylinder. It is then heated in a reduction-tube over a spirit lamp, when the metallic arsenic forming the coating is volatilised, and yields a sublimate of minute octahedral crystals of arsenious anhydride; or, if the tube be very small, or any reducing agent be added, a bright metallic ring. When the coating on the copper is sufficiently thick, it may be scraped off with a knife, and heated separately in an arsenic-tube.
This test is invaluable as affording a certain and ready means of abstracting arsenic from its solution, whether pure or mixed with organic matter. The contents of the stomach or other viscera may thus be at once examined, without any tedious preliminary operations. In this way Dr Christison discovered the presence of arsenic upwards of four months after interment; and we have ourselves found it two years and eight months after interment. The
coated copper may be preserved unharmed for years. Dr Taylor found that the 1-8th of an inch in one of these deposits that had been kept in paper nearly fourteen years gave a well-marked ring of octahedral crystals when heated.