314 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



ordinary processes. Unfortunately, the reliefs were so slightly pro- 

 nounced that the images reproduced were very confused. 



Messrs. Berres and Donne then tried to obtain blocks by attacking 

 the daguerreotype plates with aquafortis. Mr. Grove combined the two 

 foregoing methods by subjecting the plate to one of the elements of 

 a voltaic combination which acts unequally on the two metals, silver 

 and mercury. 



M. Fizeau at length designed a process which transformed daguerre- 

 otype plates into copper-plate engravings. He operated quickly 

 on the image with a mixed acid composed of nitric, nitrous, and 

 hydrochloric acids : the light spaces remained intact ; the dark 

 were affected, and an adhesive chloride of silver was formed, which 

 arrested the action of the acid. This coating was dissolved by a 

 solution of ammonia, and the action of the acid continued. To 

 obtain more depth, M. Fizeau gilded the raised parts, which were 

 thus protected from subsequent action of the nitric acid. Silver not 

 being a hard metal,, and consequently only bearing a limited amount 

 of printing, the block was coppered by galvanic processes (now, 

 copper-plate blocks are faced with steel). 



These were certainly remarkable experiments ; but, as the 

 primitive process of Daguerre was soon replaced by photography 

 on paper and on collodionized or albuminized glass, the attempts at 

 engraving daguerreotype plates were abandoned. 



Towards 1853, M. Niepce de Saint Victor obtained engravings 

 on steel in the following manner: he covered the engraving block 

 with the coating of an impressible varnish formed of bitumen, 

 benzine, wax and sulphuric ether with a few drops of oil of lavender. 

 To the plate when dry he applied a positive on paper or glass, and 

 exposed both to. the- light,, as in obtaining a proof. When the im- 

 pressed plate had passed through oil of naphtha mixed with benzine 

 it was submitted to a mixture of nitric acid and alcohol. The 

 engraving was finished off with aquafortis. 



Among the niitmerous processes since invented for printing off pho- 

 tographic proofs with printing inks, we must cite the process invented 

 by M. Poitevin, called the carbon process. We shall only briefly indicate 

 the principle of it, and we shall dwell on the results alone, because 

 this process is a part of the photographic art, nut, properly speaking 

 an application of physics : it is rather an application of chemistry.. 



