2 DR. WOLLASTON ON A METHOD 



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 

 iu the liquid, has completely subsided ; and should then be mixed with 41 parts 

 of sal ammoniac, dissolved in about 5 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 1 1 parts of platina, these, with 

 some of the other metals yet held in solution, are to be recovered, by precipi- 

 tation from the liquor with clean bars of iron, and the precipitate is to be re- 

 dissolved in a proportionate quantity of aqua regia, similar in its composition 

 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 pal- 

 ladium 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 washings. 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. 



The gray product of platina, when turned out of the crucible, if prepared 

 with due caution, will be found lightly coherent, and must then be rubbed 

 between the hands of the operator, in order to procure by the gentlest means, 

 as much as can possibly be so obtained, of metallic powder, so fine as to pass 

 through a fine lawn sieve. The coarser parts are then to be ground in a 

 wooden bowl with a wooden pestle, but on no account with any harder mate- 

 rial, capable of burnishing the particles of platina*; since eveiy degree of bur- 

 nishing will prevent the particles from cohering in the further stages of the 

 process. Since the whole will require to be well washed in clean water, the 



• The following experiment will prove the necessity of attending to this precaution : — ^if a wire of 

 platina be divided with a sharp tool in a slanting direction, and, being then heated to redness, be struck 

 upon an anvil with a hammer, so as to force into contact the two newly-divided surfaces, they will 



