1821.] On the Black Enamel obtained from Platina. 337 



sented in fig. 3 is one of the most important in this respect, as 

 the magnetism of the earth has no influence upon its position ; 

 it is the bending of the wire alone from which the needle is sus- 

 pended, which determines its direction. Another needle sus- 

 pended in the same manner, but in a horizontal position, would 

 be of great utility. By similar means, it may, perhaps, be dis- 

 covered by subterranean geometry, whether the galvanic effects 

 in mines do not disturb the magnetic needles employed. 



In another memoir, I shall endeavour to show that the circular 

 movement of electrical forces in the conductor, which I have 

 admitted as an hypothesis, results from the nature of electrical 

 forces ; and I shall also endeavour to give a new explanation to 

 the opinion which I expressed several years since, upon the pro- 

 duction of light and heat by the conflict of electrical forces. 



Article II. 



On the Black Enamel obtained from Platina. 

 By Mr. J. P. Charlton. 



(To the Editor of the Annals of Philosophy.) 



SIR, 



In my communication in the Annals for Sept. I stated the 

 grounds upon which I concluded that in all cases the rose colour 

 imparted by gold to enamels is owing, not to the oxide, as usually 

 supposed, but to the regulus of the metal ; and that the oxide, 

 where employed, is by the process of firing reduced to the 

 metallic state. Another method of procuring gold in a state of 

 fine division has since occurred to me ; viz. by separating it from 

 an alloy in which it exists in small proportion. I therefore, 

 dissolved a shilling in nitric acid, and having ground with a 

 little flux the gold which separated, in the form of a black 

 powder, obtained, as before, a rose-coloured enamel. Vitrifia- 

 tion is not essential to the production of the rose colour ; for 

 having exposed a quantity of ground flint in a crucible with a 

 little gold to a long continued and intense heat (not less than 

 110° of Wedgwood's pyrometer), I found the flint tinged 

 throughout of a delicate rose colour. 



In the same paper, I stated, that from the analogy between 

 gold, silver, and platina, and from the easy reducibility of the 

 latter, 1 was led to suppose it probable, that the black enamel 

 "Uained from platina owed its colour in like manner to the 

 regulus, and not to the oxide. Mr. Cooper's process for obtain- 

 ing the black enamel is to mix dilute muriate of platina with 

 neutral nitrate of mercury, and to expose the precipitate to a 

 hi at only sufficient to raise the calomel: the result is a black 



Neio Series, vol. n. z 



