338 O/i the Black Enamel obtained from Platina. [Nov. 



powder, which, according to him, is the protoxide of platina, 

 containing only 4 - 7 per cent, oxygen. This powder ground up 

 with flux yields a beautiful black enamel ; and Mr. C. states it 

 as a most singular fact, that the oxide, when thus mixed with 

 flux, is not reducible by the strongest heat. It appears so very 

 improbable, that the mere circumstance of being diffused in a 

 vitrified flux should enable a substance, so easily reduced as 

 platina, to retain its oxygen at intense heats, that I formed a 

 suspicion, that the platina in the black enamel must already be 

 in the metallic state. My first step was to ascertain that oxide 

 of platina, either alone or ground with flux, lost the same weight 

 in the heat of an enameller's kiln (which I found to correspond 

 with 6° or 7° of Wedgwood's pyrometer), that which was fired 

 alone appeared to have suffered no change, being still in the 

 form of a black powder. It was evidently then from this black 

 powder, whether oxide or regulus, that the colour of the enamel 

 was derived. This substance, when exposed to strong heat, lost 

 its blackness, and the platina resumed its usual metallic lustre, 

 but this change was accompanied by no loss of' weight ; conse- 

 quently the black powder was already in the metallic state, 

 however unlike in appearance to platina in its usual form. 



I find that the oxygen is entirely expelled from platina at a heat 

 below redness, but it is impossible to judge from the appearance 

 of the substance, when the expulsion begins or ends. Hence, 

 therefore, Mr. Cooper's error may be accounted for, as I con- 

 ceive that what he considered to be pure oxide, containing only 

 4*7 per cent, of oxygen, was, in fact, a mixture of oxide and of 

 regulus. 



The black grains of iridium remaining after solution of 

 platina, which, according to Mr. Tennant, are in the metallic 

 state, though friable and without lustre, are also capable of 

 imparting colour to enamel. The colour is a brownish-green, 

 when much flux is used. I am, Sir, 



Your most obedient servant, 



J. P. Charlton. 



I am uncertain whether the crystallization of metallic gold has 

 been hitherto observed or not : I beg leave, therefore, to enclose 

 a few minute specimens; they were obtained by boiling nitric 

 acid upon mercury, which happened to hold a little gold in solu- 

 tion; when the mercury was completely dissolved, long crystal- 

 lized filaments remained : they were much broken in extracting 

 them from the matrass. 



