Platina and Mercury upon each biker. 105 



triple salt of platina and mercury shall be presently exa- 

 mined. From this it is evident that to produce the union 

 of platina and mercury, the latter being at its minimum 

 of oxidizement in nitric acid, the addition of green sulphate 

 of iron is superfluous. 



But if mercury be raised to its maximum of oxidizement 

 in nitric acid the case is diticrent, for no precipitation occurs 

 till the green sulphate of iron is added. The most advan- 

 tageous method for precipitating platina and mercury by 

 green sulphate of iron is, I believe, the following. Mix a 

 solution of platina with. a solution of green sulphate of iron, 

 both warm, and add to them a solution of nitrate of mer- 

 cury at the maximum of oxidizement, also warm. It is 

 necessary to avoid excess of acid, salt, &c. in this as in all 

 such cases. With due care the precipitation of both metals 

 will then be complete. 



By comparing the experiments made with mercur\' and 

 platina with those made with silver and platina, a striking 

 resemblance will be found. This induced me to pursue the 

 analogv, and to examine whether, independently of the ac- 

 tion of platina"^ mercury had not the saine property of being 

 precipitated by creen sulphate of iron as silver. Nitrate of 

 silver is precipitated by green sulphate of iron, but muriate 

 of silver is not sensibly acted upon by the same reagent. 

 The insolubility of muriate of silver might be alleged as the 

 cause of this, if I had not tried the experiment by pouring 

 nitrate of silver into green muriate of iron, m which case 

 all the substances were presented to each other in solution. 

 The result was not reduction, but muriate of silver and ni- 

 trate of iron. This fact rests upon a much more extensivb 

 basis than mere mechanical circumstances ; and, if pursued 

 with the attention it deserves, it would lead us into the 

 wide expanse of coni plicated affinities and their relations. 

 From reasoning alone wc should ba disposed to think that 

 an acid, so easily decomposed as the nitric, would be suf- 

 ficient to prevent the reduction of a metal which it can dis- 

 solve. But on the one hand it can spend its oxygen upon 

 a part of the oxide of the green sulphate of iron, while on 

 the other its affinity for oxide of silver is not powerful 

 enough to retain it, when there is another part of the oxide 

 of iron present to deprive it of oxygen. But the affinity of 

 piuriatic acid for oxide of silver, one of the strongest at pre- 

 sent known, is sufficient to counterbalance all the other 

 forces. There are many other instances ot the same kind. 



If then a solution of green sidptiate of iron be Iwout^lit 

 into contact with either aoluble or insoluble muriate Of 



mercur)', 



