NATURAL PHILOSOPHY. 183 



much encouraged by my experiments, and have strong hopes of being able 

 to substitute this process for a considerable portion of the engraving, if not 

 to make a complete map. My examination has not yet extended to the pro- 

 cesses of Xiepce and Talbot, except that I have determined that their processes 

 can be worked on copper, which is much more economical than steel, as the 

 plates of copper can be prepared by the electrotype process of perfect surface, 

 while expensive manual labor only can furnish those of steel. 



- M. Donne proposed to etch a Daguerreotype with nitric acid. The 

 chemical difference between the ' lights' and ' shades' of the Daguerreotype 

 appears to be sufficient to determine the action of the acid to the dark parts, 

 which are supposed to be silver, while the lights are mercury. I found this 

 process extremely uncertain. In twenty carefully-conducted experiments, I 

 succeeded but once in getting definite markings, and this only on a portion 

 of the plate experimented on : hence, I conclude the process is not likely ever 

 to be turned to account. Professor Grove proposed to engrave the Daguerreotype 

 plate by electro-etching it, making it the positive electrode in a strong bath 

 of chlorohydric acid. This process gives engraving of great beauty, but, un- 

 fortunately, of very great delicacy. I made over a hundred trials of this pro- 

 cess, and, although I generally obtained a beautiful engraving, yet in only 

 two instances did I obtain lines of sufficient depth to print from, and then the 

 quantity of ink which could be held in the line was only sufficient to give a 

 mere stain on the paper, instead of a well-defined black line. M. Gaudin pro- 

 posed to conduct the process of M. Donne on a plate of steel, covered with a 

 film of silver, to enable it to receive the Daguerrean image. This was de- 

 signed for producing the graving on a more durable material than silver. But 

 M. Gaudin does not state whether he designed merely to communicate a silver 

 character to the surface of the steel, to enable it to be used as a Daguerrean 

 plate ; or whether he designed such a coating, that while it might be eaten 

 through in the shades, to expose the easily-oxidable steel, the lights would be 

 protected by the silver film. If the former were his design, I must conclude, 

 from my experiments, that it is impossible, because an infinitely thin coating 

 of noble metal over an oxidable one, determines the formation of multitudes 

 of small galvanic circles on the surface. 



"I have long had the idea of silvering a copper plate just sufficiently thick 

 to defend the copper from such chemical action as would corrode it, and of 

 then eating through this film by the process of Professor Grove, and afterward 

 well biting the copper in the bared parts. This I have great expectation of 

 determining to be a practicable method of producing a chemiglyphic line- 

 engraving ; but the essential idea of this is probably due to M. Gaudin, instead 

 of myself. 



"The thickness of the film of silver required for the protection of the base 

 metal I have found to be quite considerable ; and this, combined with the very 

 slight depth to which the silver can be etched, makes the problem of obtaining 

 an engraved plate by the method I propose, one of great delicacy. Unfortu- 

 nately, the requisite thickness of the film is nearly equal to the greatest depth 

 of bite that can be obtained ; and hence we may have a good Grove's etching 

 on the film, yet the base metal only partially denuded ; and then, if we at- 



