VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 521 



4. Deflagration of Zinc with Ptatina. 



A mixture of platina and zinc, exposed to a strong fire, deflagrated, and ap- 

 peared in violent agitation. This continued but a little time ; the matter quickly 

 became solid, and could no longer be made to flow; or the zinc, of which a 

 considerable proportion remained in it, to flame. The mass was very brittle, 

 dull-coloured, spongy, and of no specific gravity. 



General Remarks. 



This extraordinary mineral, on which the most active fluxes, assisted by the 

 most intense fires, have no effect, melts perfectly with all the known metallic 

 bodies ; unless arsenic, a substance impatient of a degree of heat sufficient to 

 render itself fluid, is an exception.* All the metals take up equal their own 

 weight ; some metallic compositions more than twice their weight. 



Platina appears in general to have no remarkable affinity with one metal more 

 than with another. Lead and iron, which do not mingle together, and of which 

 the former will take up some bodies from the latter, and the latter some from 

 the former, seem both indifferent to platina; which, if combined with either, is 

 not separated by the other. 



Yet some substances have greater or less degrees of affinity with platina, than 

 with other metallic bodies. Thus, from aqua regia, in certain circumstances, it 

 throws out gold ; and is itself precipitated by the other metals, which dissolve in 

 that menstruum. From quicksilver it throws out lead ; and is itself thrown out 

 by gold. 



The changes which platina occasions in the perfect metals, were examined in 

 a former paper : its effects on the semi-metals are less I'cmarkable. The princi- 

 pal are, that it increases the hardness of zinc, and the antimonial semimetal, 

 but not of bismuth ; and disposes this last to change its colour in the air, but not 

 the others. 



Its effects on the compound metals, are similar to those which it produces on 

 the simple ones. Brass it renders white, hard, brittle, susceptible of a fine 

 polish, and not liable to tarnish in the air, as it does the copper, and in Some 

 degree the zinc, of which this metal is composed. Mixtures of it with copper 

 and tin are more apt to tarnish than with copper only, and less than with tin only. 



All metallic substances, except gold, are exeded from platina by the simple 

 acids : mercury is the only one separable by fire. The platina remaining after 

 the separation of the metals, proves unfusible as at first. 



Platina perfectly resists the destructive power of lead and bismuth, and the 



* As mentioned in a former note, it may be brought into fusion when subjected to a suflScicnt 

 degree of heat, with white arsenic, .^ , ., 



VOL. X. 3 X 



