196 ANNUAL OF SCIENTIFIC DISCOVERY. 



plate. This coating must be kept very thin, lest the fineness of the 

 engraving and the polish of the white parts, should be destroyed. In 

 this state the plate can be delivered to the printer. 



After a certain number of impressions have been obtained, it will 

 be perceived that the coating of copper is worn in some places ; 

 then this coating must be removed, and a fresh one applied in its 

 place. For this purpose, the plate must be purified and cleansed by 

 warm potash, and plunged in a weak acid composed as follows : Wa- 

 ter, 600 parts ; nitric acid, 60 parts ; nitrous acid of engravers, 5 parts ; 

 all in one volume. This acid will dissolve the coating of copper, and 

 the plate being coppered again by the same means as before, may 

 be again admitted to the operation of printing ; and as nothing can 

 prevent the success of a repetition of the same operation, any number 

 of impressions may be obtained. The coating of the copper can also 

 be removed by caustic ammonia. 



The daguerreotype plates engraved by this process, constitutes the 

 present invention, which consists, 



First, In the discovery and employment of certain properties of a 

 mixture composed of nitric acid, nitrous acid, and hydrochloric acid 

 in determined or fixed proportions. The two last mentioned acids 

 may be employed either in a free state, or combined with alkaline or 

 other bases. This mixed acid has the property of biting the pure 

 silver which forms the black parts of the daguerreotype picture, with- 

 out attacking the white parts forming the amalgam of mercury. The 

 result of the action of biting is to form on the black parts of the pic- 

 ture an insoluble chloride of silver ; and this chloride of silver, which 

 when formed, stops the action of the acid, is dissolved by ammonia, 

 which allows the biting to continue. 



Secondly, In the discovery of certain properties of a warm solu- 

 tion of caustic potash, and in the employment of the said solution, by 

 which the mercury forming the picture is better and deeper amalga- 

 mated with the silver under it, so that many imperceptible points of 

 the amalgam are effected in such a manner that the acid has no action 

 upon them. 



Thirdly, In the discovery and employmeut of a process which 

 produces a grain favorable to the engraving, by which the biting on 

 the plate is rendered deeper. This is effected by filling the parts 

 engraved with a siccative ink, or any substance, and then gilding the 

 plate by the electrotype process ; the gold is not deposited on the 

 parts protected by the ink. When the plate is gilded, the ink is 

 cleansed by the caustic potash, and the plate may be submitted to the 

 effects of an acid which does not attack the coating of gold, but bites 

 only on the silver in the parts already engraved by the first operation. 



Fourthly, In the employment of a process by which the plate is 

 protected from the wear of the printing operation. This is effected 

 by covering the plate, before printing, with a slight coating of copper 

 by the electrotype process ; and when the coating begins to wear by 

 printing, it is removed by weak acid, or by ammonia which dissolves 

 the copper without affecting the silver under it. The plate is cop- 



