From the general use of common salt, the chemist must be prepared to meet with a _muriate_ in almost every examination after arsenic, besides which this latter substance is occasionally adulterated with the _muriate of baryta_ and by _sulphate of lime_. _Dr. Marcet_ proposes to obviate the difficulties which the presence of a _muriate_ must occasion, by adding to the fluid to be examined dilute _nitric_ acid, and then cautiously applying the _nitrate of silver_ until all precipitation ceases; in this way the muriatic acid will be entirely removed, while the arsenic, if present, will be retained in solution, and may be afterwards rendered evident by the affusion of ammonia, which will instantly produce the yellow precipitate in its characteristic form. It must, however, be confessed, that this mode appears complicated, and, moreover, requires some chemical address for its accomplishment; it should be also known that the yellow precipitate thus produced is not always permanent, for it is soluble in an excess of _ammonia_. Under these circumstances, it is surely preferable to precipitate at once from the fluid under examination, all the substances which nitrate of silver can affect, and then to expose the mixed and ambiguous precipitate, so obtained, to a low heat, in a glass tube, when the arsenious acid will be separated by sublimation. In this way the presence of _muriates_ and even _phosphates_, may, in certain cases, be serviceable, especially if the quantity of arsenic be very minute; for, by increasing the bulk of the precipitate, we shall decrease the difficulty of its examination. OBJECTION 3. _Chromate of potass produces with nitrate of silver a yellow precipitate, which, when placed side by side with one produced by arsenious acid, cannot be distinguished by colour or appearance from it._ This fact has lately been announced by Dr. Porter, of the University of South Carolina (_Silliman’s Journal_, _iii._ 355); but as the presence of _Chromate of Potass_ can never be suspected in any research after arsenic, in cases of forensic interest, it is unnecessary to enter into any details respecting it. We have stated above, that in consequence of the inability of arsenious acid to decompose _nitrate of silver_ by simple elective attraction, the presence of _some_ alkali becomes indispensable in the examination; and for this purpose _Dr. Marcet_ suggested the superior advantages which would attend the application of _ammonia_, in all those cases where the arsenic had not been previously combined with a fixed alkali; since the former does not, when added singly, decompose nitrate of silver; a circumstance which, in using the fixed alkalies, is very liable to occasion fallacy. This led _Mr. Hume_ to improve his original plan, by forming at once a compound,[254] which he calls the _Ammoniaco-nitrate of silver_, but which may with more propriety be designated, as an _ammoniuret_ of that metal.