OBJECTION 1. _The alkaline Phosphates are found to produce precipitates with silver, analogous in colour and appearance to the arsenite of silver._ This constituted one of the principal points in the evidence for the defence, on the trial of _Donnall_ for the murder of _Mrs. Downing_ (_see Appendix_, p. 299), and it must be admitted as a valid objection, if the experiment be performed in the manner just stated; but there are other reagents which will immediately distinguish these bodies, as we shall presently have occasion to state, under the history of the _Ammoniuret of silver_, as a test for arsenic. The author has also shewn that there is a mode of so modifying the application of the present test, that no error or doubt can arise in the use of it, from the presence of any phosphoric salt. This method consists in conducting the trial on writing paper, instead of in glasses; thus—drop the suspected fluid on a piece of white paper, making with it a broad line; along this line a stick of _lunar caustic_ is to be slowly drawn several times successively, when a streak is produced of a colour resembling that known by the name of _Indian Yellow_; and this is equally produced by the presence of arsenic, and that of an alkaline phosphate, but the one from the former is rough, curdy, and flocculent, as if effected by a crayon, that from the latter is homogeneous and uniform, resembling a water-colour laid smoothly on with a brush; but a more important and distinctive peculiarity soon succeeds, for, in less than two minutes the phosphoric yellow fades into a _sad green_, and becomes gradually darker, and ultimately quite black; while, on the other hand, the arsenical yellow remains permanent, or nearly so, for some time, when it becomes brown. In performing this experiment the sun-shine should be avoided, or the transitions of colour will take place too rapidly. It would be also prudent for the inexperienced operator to perform a similar experiment on a fluid known to contain arsenic, and on another with a phosphoric salt, as a standard of comparison. In this way the _nitrate of silver_, without the intervention of any other test, is capable of removing every ambiguity, and of furnishing a distinguishing mark between the chemical action of arsenic and that of the phosphates. _Mr. Hume_[253] states that he has repeated this modification of his experiment with entire satisfaction; and that, in a late unfortunate case of poisoning, he derived considerable information by its application. One of the great advantages of this test is the very small quantity that is required for examination, and which will therefore never prevent our pursuing the subject through the other channels of investigation. OBJECTION 2. _The muriates produce precipitates with silver, so copious and flocculent, as to overcome every indication which the presence of arsenic would otherwise afford._