27 



exposed to the vapour of this substance for at least half the neces- 

 sary time; for the application of the layer of gold is opposed to 

 the formation of what is called the veil of bromine. This facility 

 must not, however, be abused ; for the layer of gold, being very thin, 

 might be attacked, especially if too much polished. The process 

 which I am about to give may, perhaps, be found rather com- 

 plicated ; but, notwithstanding my desire to simplify it as much as 

 possible, I have been led, on the contrary, by the results of my 

 experiment, to multiply the substances employed, all of which play 

 an important part in the whole process. I regard them all as 

 necessary for obtaining a complete result, which must be the case, 

 since I have only gradually arrived- at discovering the properties of 

 these different metals, one of which aids in promptitude, the other 

 in the vigour of the impression, etc. 



From the concurrence of these substances, arises a power which 

 neutralises all the unknown effects which so often oppose the 

 formation of the image. 



I think, besides, that science and art should not be interrupted by 

 the consideration of a more or less long manipulation ; we should be 

 contented to obtain beautiful results at this price, especially when 

 the means of execution are easy. 



For the galvanic preparation of the plate does not present any 

 difficulty. The operation is divided into two principal parts : the 

 first, which is the longest, may be made a long time previously, 

 and may be regarded as the completion of the manufacture of the 

 plate. This operation, being once made, serves indefinitely ; and, 

 without recommencing it, a great number of impressions may 

 be made on the same plate. 



PREPARATION OF THE NEW SUBSTANCES. 



Aqueous Solution of Bichforide of Mercury. 5 decigrammes of 

 bichloride of mercury in 700 grammes of distilled water. 



Solution of Cyanide of Mercury. A flask of distilled water is 

 saturated with cyanide of mercury, and a certain quantity is 

 decanted, which is diluted by an equal quantity of distilled water. 



Acidulated White Oil of Petroleum. This oil is acidulated by 

 mixing with it one-tenth of pure nitric acid, leaving it at least 

 forty-eight honrs, occasionally agitating the flask. The oil, which 

 is acidulated, and which then powerfully reddens litmus paper, is 

 decanted. It is also a little coloured, but remains very limpid. 



Solution of Chloride of Gold and Platinum. In order not to 



