Cofitrilution towards the assaying of Coins » 259 



ind precipitated as before, by iron, I mixed the solution 

 to complete saturation with caustic ammonia, which again 

 dissolved the precipitated calx, and the calx of iron re- 

 mained behind. When the latter had been separated, 1 

 saturated the superabundant ammonia with sulphuric acid, 

 and precipitated the calx of tin by mild potash. 



Exp. III. I employed a shorter process with the third 

 portion of the solution containing zinc. I evaporated it to 

 dryness, rcdiesolved the mass in diluted sulphuric acid, pre- 

 cipitated the copper by a rod of zinc accurately weighed, and 

 after its separation, precipitated the dissolved zinc by mild 

 potash ; and from, the quantity of the calx of zinc which I 

 found, I deducted that portion which the rod of zinc em- 

 ployed for the precipiiation had contributed. 



Exp. IV. In regard to the fourth portion I endeavoured 

 to accomplish what I had in view by the following means : 

 I diluted it with four parts of water and poured it into a flat 

 dish, the bottom of which was covered by a thin plate of 

 lead. At the end of a few days I found the solution com- 

 pletely decomposed, and the copper precipitated in a me- 

 tallic form without being rendered impure by a quantity of 

 precipitated metallic calx, as is commonly the case when 

 zinc and iron are employed for the precipitation of copper 

 from pure nitric acid. After the copper was separated, I 

 reduced the fluid, which was of a straw colour, to a small 

 quantity by evaporation, mixed it with a saturated solution 

 of sulphate of soda, separated from it the sulphate of lead 

 which was formed, and precipitated the pure zinc by car- 

 bonate of potash. Of these four ways of detecting tke tin 

 contained in the mixture, the last appeared to me to be the 

 best : I therefore employed it chiefly in my examination of 

 coins. 



That the calx of zinc which I obtained was actually that 

 substance I assured myself by mixing it v/ith charcoal 

 powder, putting small plates of copper along with it in a 

 covered crucible and bringing it to a proper degree of heat. 

 After cooling, I found that the copper was converted into 

 brass. 



I dissolved another part in acetous acid, and left the so- 

 lution to spontaneous evaporation, by which means the 

 ucetite of zinc shot into that cr\stalline form peculiar to it, 

 that i:; to ';ay, tablets of six planes with equal angles. 



The solutions of the Roman coins after the separation of 



the zinc, when any of that metal was present, I divided into 



two parts. I employed one of them to examine whether 



R 2 they 



