_b._ The _Ammoniuret of Silver_. This is an improvement of considerable value; for, while it obviates the necessity of ascertaining the exact proportion[255] of alkali required in each experiment, it possesses the desirable property of not in the least disturbing the solution of _phosphate of soda_. _c._ _Sulphate of Copper._ This test of arsenic is the one discovered by _Scheele_; when added to the _arsenite of potass_ a beautiful green precipitate (constituting a pigment known by the name of _Scheele’s green_) is produced; “so decidedly,” says _Dr. Bostock_, “does this phenomenon indicate the presence of arsenic, that I thought it desirable to ascertain, as exactly as possible, what were the best proportions in which the ingredients should be employed, and in what way they should be mixed, so as to exhibit the effect in the most obvious manner. After a number of trials, in which the substances were employed in various quantities, and under different circumstances, I am disposed to recommend that the proportions of the _arsenic_, the _potass_, and the _sulphate of copper_, should be to each other as the numbers _one_, _three_, and _five_, respectively; for instance, if one grain of arsenic and three grains of potass, be dissolved in two drachms of water; and, in another equal quantity of water, five grains of sulphate be dissolved, we have two solutions, which are transparent, and nearly colourless; but upon mixing them together, the whole is converted into the most beautiful grass-green, from which a copious precipitate of the same hue slowly subsides, leaving the supernatant fluid nearly without colour. If the same materials are employed, in the same manner, but without the arsenic, a delicate _sky-blue_ is formed, which is so decidedly different from the former colour as not to admit of the possibility of error.” In this experiment then, as well as in that with the nitrate of silver, it is necessary that the arsenious acid should be combined with an alkaline base; and for the same reason, in order to bring the double elective attractions into play; _Mr. Hume_ has accordingly availed himself of the property of ammonia, to form an _ammoniuret of copper_, which is to be made according to the formula already given for the preparation of the silver test. _d._ _Ammoniuret of Copper._ In using this test care must be taken that it be not too highly concentrated, for in that state it will not produce precipitation.