2 Dr. WoUaston ow a Method 



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, sus- 

 pended in the liquid, has completely subsided; and should 

 then be mixed M'ith 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 1 1 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 mu- 

 riatic acid should be mixed with 32 parts by measiu'e 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 con- 

 tained in the complicated ore in question ; antl must ultimately 

 be well pressed, in order to remove the last renmant 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 cruci- 

 ble, if pi'epared with due caution, will be found lightly cohe- 

 rent, and must then be rubbed between the hands of the ope- 

 rator, 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 material, capable of burnishing the 

 particles of platina*; since every degree of burnishing will 



prevent 



* The following cxpcnnient 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, beint; 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 become firmly welded together ; but if the surfaces liave previously 



been 



