Chap. 36.] Natural Hiftory of Platina. 239 



they exceed it. The fpecific gravity of gold is about 

 nineteen times that of water, whereas platina, which 

 ftill contains fo much iron as to render it magnetical, 

 is upwards of twenty-one times the weight of that 

 fluid. It is extremely difficult to free platina from 

 the laft portions of iron, but fome minute particles, 

 which have be>n fufed by the focus of a burning glafs; 

 and fo far purified as not to be attracted by the mag- 

 net, appear to exceed twenty-two times the weight of 

 water. 



Platina is, perhaps, the mod perfect of all the me- 

 tals. As it fo confiderably exceed^ even gold in weight, 

 it is therefore to be confklered as the moft ponderous 

 body in nature. 



It has feveral properties in common with the moft 

 ufeful of metals, iron. In hardnefs it approaches to 

 that metal in the ilate of fteel ; and in infufibility it 

 exceeds it even in the ftate of foft iron ; it alfo confi- 

 derably refembles iron in appearance, and it is th* 

 only metal, befides iron, which has the property of 

 welding. 



Platina refembles gold in being fallible only in aqua 

 regia, and it even requires a larger quantity of that 

 compound acid for its folution than gold. The folu- 

 tion is of a deep yellow or reddifh colour. The pro- 

 portions of acids beft adapted to the folution of platina, 

 are equal parts of the nitrous and muriatic acids; but 

 the folution does not then take place with rapidity. 

 This compound is very corrofive, ana tinges animal 

 fubftances of a blackifh brown colour. 



The vegetable alkali added to this folution^ only oc- 

 cafions the precipitation of a part of the metal in the 

 form of a fparkling powder, which is foluble in a large 

 quantity of water. A very remarkable circumflance 

 is, that the foffil alkali does not produce any precipi- 

 tation, 



