98 Dr. Wollaston on a Method of 



should be diluted. I now therefore recommend, that to every 

 measure of the strongest muriatic acid employed, there be 

 added an equal measure of water ; and moreover, that the nitric 

 acid used be what is called u single aquafortis ;" as well for the 

 sake of obtaining a purer result, as of economy in the purchase 

 of nitric acid. 



With regard to the proportions in which the acids are to be 

 used, I may say, in round numbers, that muriatic acid, equi- 

 valent to 150 marble, together with nitric acid equivalent to 

 40 marble, will take 100 of crude platina ; but in order to 

 avoid waste of acid, and also to render the solution purer, 

 there should be in the menstruum a redundance of 20 per 

 cent, at least of the ore. The acids should be allowed to digest 

 three or four days, with a heat which ought gradually to be 

 raised. The solution, being then poured off, should be 

 suffered to stand until a quantity of fine pulverulent ore of 

 iridium, suspended in the liquid, has completely subsided ; and 

 should then be mixed with 41 parts of sal ammoniac, dissolved 

 in about five times their weight of water. The first precipitate, 

 which will thus be obtained, will weigh about 165 parts, and 

 will yield about 66 parts of pure platina. 



As the mother-liquor will still contain about 11 parts of 

 platina, these, with some of the other metals yet held in solu- 

 tion, are to be recovered, by precipitation from the liquor with 

 clean bars of iron, and the precipitate is to be redissolved in a 

 proportionate quantity of aqua regia, similar in its composi- 

 tion to that above directed to be used : but in this case, before 

 adding sal-ammoniac, about 1 part by measure of strong 

 muriatic acid should be mixed with 32 parts by measure of the 

 nitro-muriatic solution, to prevent any precipitation of palla- 

 dium or lead along with the ammonio-muriate of platina. 



The yellow precipitate must be well washed, in order to free 

 it from the various impurities which are known to be contained 

 in the complicated ore in question ; and must ultimately be 

 well pressed, in order to remove the last remnant of the wash- 

 ings. It is next to be heated, with the utmost caution, in a 

 black-lead pot, with so low a heat as just to expel the whole 

 of the sal-ammoniac, and to occasion the particles of platina 

 to cohere as little as possible ; for on this depends the ultimate 

 ductility of the product. 



