1819.] for Painting in Oil. 363 



mixture of oils or gum renders it sufficier/fly dark ; and expe- 

 rience has shown that it will answer for all the occasions which 

 paiuters can require. 



I have in vain attempted to obtain a colour of greater intensity 

 by increasing the proportion of gold. The shade acquires more 

 of the violet as it becomes deeper, as may be seen in the speci- 

 men (6), which may be employed with advantage in painting the 

 shades . In general, the violet-purple tints are very easily obtained ; 

 all the earths furnish them ; but it is very difficult to have the 

 true purple of the specimen («) by any other process than the 

 one which I have described. 



For oil painting, this colour must be carefully rubbed with a. 

 mixture of drying oil and varnish. The painting is to be begun 

 by a thin transparent coat. A second coat is sufficient to give 

 it all the lustre of which it is susceptible — a lustre which is equal 

 to the ordinary cochineal lake. The under coats {les dessous) 

 ought to be prepared with rough terra de Sienna.- 



This unalterable colour is particularly useful in miniature 

 painting, and mav be usedinstead of cochineallakes in carnations. 

 A mixture of it with vermilion gives beautiful tints ; and light,, 

 which gradually destroys the light shades of carmine, has no 

 effect whatever upon gold purple, which is capable of resisting 

 both the action of fire and of light. 



A detailed memoir of these experiments was presented in 1818 

 to the Academy of Sciences of Turin, and it has been ordered 

 to be printed in the last volume of its collection of memoirs. 



Accept, Sir, the assurance of the most distinguished consider- 

 ation with which I have the honour to be, Sir, 



Your very humble and obedient servant, 



The Count de Maistre. 



Article VIII. 



Some Observations on the Action of Nitric Acid on Lithic or 

 Uric Acid, in liephj to^.M. Vauquetiu. By W. Prout, M.D. 

 F.R.S. &c. 



Upwards of twelve months ago, I was informed that M. Vau- 

 quelin had been repeating my experiments on lithic acid, and 

 had not been able to succeed in forming purpuric acid. Since 

 that time, 1 have been waiting with some degree cf impatience 

 to see what this celebrated chemist had to say on the subject. 

 At length a notice has appeared in the last number of the Royal 

 Institution Journal, to which 1 now purpose briefly to reply. 



In the first place, M- Vauquelin denominates me a repealer of 

 M. Brugnatelli's experiments — a term calculated to convey an 



