186 ANNUAL OF SCIENTIFIC DISCOVERY. 



dercut in an irregular manner, and blended together, and the broad lines are 

 smooth on the bottom. 



" But all these difficulties were duly considered in the Coast Survey Office, 

 in the very beginning, and had there not appeared at least a prospect of over- 

 coming them, the announcement had never been made that we were en- 

 deavoring to apply actino-engraving to facilitate the production of our charts. 

 The difficulty has not, indeed, been wholly overcome ; for then the whole 

 problem of actino-engraving were completed; yet it has been greatly reduced, 

 and what remains of it does not appear of such magnitude but that it may 

 be overlooked, at least in the production of the coarser kinds of engraving. 



" First of all, it has been found that the great irregularity, undercutting, and 

 spreading of the lines in the common etching process, are due to the peculiar 

 nature of the solvent (nitric acid) employed, and the irregular texture of the 

 metal ; which irregularity causes one part to serve in an electrical relation to 

 another ; thus thousands of points and lines of action are formed, other than 

 "those of the design. Again, nitric acid is an electrolyte, just the thing to 

 concur with the irregular texture for the production of irregular action ; and 

 the nitric acid, in acting on the metal, is decomposed into several substances, 

 which have various actions on the metal. If a clean line is once begun, why 

 should not the solvent act on it uniformly on all the parts? If it does not, 

 there are reasons for it, and those reasons I have given you above. I indeed 

 am aware that 'to find the cause of a disease is not to find the cure,' but still 

 it is to find where the cure is to be applied. 



" In the Daguerreotype, the mercury is determined to those portions of the 

 plate where the light has acted, producing a drawing in mercury on a silver 

 ground. The chemical difference between mercury and silver, with respect 

 to nitric or hydrochloric acid, has been the fundamental idea in all the pre- 

 vious attempts to engrave the Daguerreotype ; the mercury being noble to 

 silver, it was supposed that it would serve the part of the common etching 

 ground, to limit the action of the acid. But the chemical difference between 

 silver and mercury is not at all sufficient for this purpose. Silver forms but 

 an extremely feeble voltaic combination with mercury ; and the two metals 

 behave very nearly alike with nitric or hydrochloric acid. I have now de- 

 termined, ha the most indisputable manner, that the formation of the etching 

 is not at all owing to the mercury acting as a protecting film. Let a strong- 

 Daguerreotype be made, and finished in the usual method, except the gilding ; 

 then let the design be rubbed out, and the plate well buffed and galvanized : 

 Now let the plate be connected with the battery for etching, after the manner 

 proposed by Professor Grove, and, lo ! the design will appear by being etched. 

 Where now is the protecting film of mercury? Any mode of etching 

 founded on such an idea is but a chimera, and must necessarily fail. 



"I have come to the conclusion that the Daguerrean image is molecular, 

 and not chemical, in its production and constitution. What do you think of 

 a picture all the way through the plate, and the plate wholly formed of pure 

 copper? I can easily produce such now. In the lights of the picture the 

 silver is cristallized, and the cristallization is proportional to the intensity 

 and duration of the light. The action of the light on the halogenized sur- 



