PHILOSOPHICAL TRANSACTIONS. [aNNO 1780. 



if it be dissolved in such acid diluted with water, the arsenic will tall to the 

 bottom in a white powder or crystals, and the solution being poured off will 

 contain the metal, which may be separated from the acid by another metal as 

 before. 



3°. The calces of metals may be dissolved in acids, whether they be pure (of 

 which there are few instances in ores) or combined with gas, respirable air, or 

 other vapours; therefore, if the metal in an ore be in the form of a calx, we may 

 find an acid which will dissolve it, and we may afterwards precipitate it in its 

 metallic form as before. 



4°. When an ore is to be assayed, it should be separated from the quartz, 

 spars, and other earthy matters, with which it is often mixed, as perfectly as 

 possible; however, after all our care there will be often a part of them so inti- 

 mately mixed with the ore, that it cannot be entirely cleared. Many of these 

 earthy matters do not dissolve readily in acids: therefore, if the metal of an ore 

 be dissolved in an acid, so as to form a compound soluble ii< water, the solution 

 of the metal may be poured off, leaving such earthy matters behind. 



5°. If the earthy matter should dissolve in the acid, it is seldom to be precipitated 

 by a metal: therefore if both earth and metal be dissolved, on the application of 

 another metal, which attracts the acid more strongly, that which was combined 

 with the acid will be precipitated, and the earth left in the solution. 



6°. Acids attract the metals with different powers: therefore, if two metals 

 be combined with an acid, and we apply to the solution a mass of that which 

 attracts the acid strongest, the other will be precipitated. The mass being 

 weighed before and after the precipitation, the difference will be the quantity of 

 additional metal dissolved; if therefore we pour off the liquid from the precipi- 

 tate, and apply another metal, which attracts the acid still more strongly, the 2d 

 metal will be precipitated, which being weighed, and the weight lost from the 

 mass deducted, gives the weight of the 2d metal. As this principle is of great 

 use in investigating the elements of mixed metals, we shall give an example. 

 Suppose copper and silver mixed; dissolve the whole in pure nitrous acid, pro- 

 perly diluted with water; apply to the solution a mass of copper, the silver will 

 be precipitated. Pour off the solution, and wash the silver and undissolved 

 copper with water; pour the washings into the solution; weigh the mass of 

 copper left, and mark what it has lost; apply to the solution a mass of iron, 

 the whole copper will be precipitated. Pour off the fluid, and wash the precipitate 

 carefully, dry it, and weigh it, deduct the weight lost from the mass of copper, 

 what remains is the weight of the copper in the mixture; if this weight, toge- 

 ther with that of the silver, be the weight of the metal originally exposed to 

 examination, there is no reason to suspect any mixture of another metal. 



If the metals mixed are unknown, and we can find an acid which will dissolve 



