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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 : 

 Water, 600 parts ; nitric acid, 50 parts ; nitrous acid of engravers, 

 5 parts; all in volume. This acid will dissolve the coating of 

 copper, and the plate being coppered again by the same means as 

 before, may be again submitted to the operation of printing; and as 

 nothing can prevent the success of a repetition of the same opera- 

 tion, any number of impressions may be obtained. The coating of 

 copper can also be removed by caustic ammonia. 



The Daguerrotype plate engraved by this process, which consti- 

 tute the present invention, consist,- 



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 basis. This mixed acid has the property of biting 

 the pure silver which forms the black parts of the Daguerrotype 

 picture, without attacking the white parts formed by the amalgam 

 of mercury. The result of the action of the biting is to form on the 

 black parts of the picture 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 

 solution of caustic potash, and in the employment of the said solu- 

 tion, by which the mercury forming the picture is better and deeper 

 amalgamated 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 employment of a process which 

 produces a grain favourable 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 other substance, and then 

 gilding the plate by the electrotype process; the gold is not de- 

 posited 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 sub- 

 mitted 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 sl.glit coating of copper 



