NATURAL PHILOSOPHY. 185 



been sufficient to hold the full quantity of ink required to give a clear black 

 print. "When nitrate of silver is used for the second biting, the tendency to 

 open the pores of the film is not so great as when the iron salts are used ; but 

 this solvent has a very great tendency to deposit the reduced silver in the fine 

 lines, and this even closes the pores of the film ; but, for coarse lines. I have 

 thought that it made a cleaner plate than the iron salts. I have sought to 

 obtain a third biting, by filling the lines obtained by the second biting with 

 some non-conducting substance, and then heavily silvering or gilding the lights, 

 dissolving out the non-conducting substance from the lines, and again biting ; 

 but every experiment I have made in this direction has failed for the following 

 reasons : the only salts of silver and gold which can be successfully used 

 for electro-deposition are alkaline ; the alkali, acting on the oil, gum, resin, 

 wax, or other hydro-carbon which fills the line, dissolves it before the lights 

 are sufficiently coated. Again, when the non-conducting film in the lines is 

 very thin, it is ruptured by the affinity which the negative element of the 

 electrolyte has for the copper. I find that asphaltum is not very readily acted 

 on by the alkaline salts of the noble metals, but I have not yet succeeded hi 

 working this substance into the lines. 



" I took an alto by the electrotype process, from a plate, after the first biting, 

 and gave the alto a good coat of gold, and ground down the raised fines with 

 a soft piece of charcoal, so as to cut through the gilding and expose the cop- 

 per. The plate was then bitten with the perchloride of iron : all the fine lines, 

 as the hair-strokes of the letters and figures, were destroyed, but a good solid 

 cutting of-the coarser parts was obtained. In putting on the silver, I judged 

 of the thickness and weight by making the current which reduced the silver 

 pass through a voltameter in the same circuit, and carefully noting the volume 

 of hydrogen evolved, and referring to the respective equivalents and specific 

 gravities. This is a rather laborious process, but it is probably the only one 

 by which small quantities of electro-deposited metal can be measured. 



Some of the trial maps for the Coast Survey printed from Mr. Mathiot's 

 plates are perfect in detail, yet deficient hi vigor, which is the only thing re- 

 quired to make them engravings of the very first order. As it is, some of the 

 plates, executed in the short space of eighteen minutes, from drawings on 

 paper, are, with a slight retouching by the graver, sufficiently perfect for use ; 

 thus producing an immense saving hi time, labor, and expense. Mr. Mathiot 

 is still prosecuting his experiments under the direction of the Coast Survey 

 Office, and further interesting results may be expected. Editor. 



Some fears having been expressed by the editor of Humphrey's Photo- 

 graphic Journal hi regard to this process, "that the action of the acid in 

 biting in the copper in the barred parts, would also proceed laterally as well 

 as deeply ; and thereby breaking up the superficial coating, whether of silver 

 or gold, and producing rough and uneven lines," Mr. Mathiot, in a reply, pub- 

 lished in the Journal, Xov. 15th, 1855, says: "These objections are indeed 

 formidable. I believe these difficulties have confronted every one who has 

 thought of actino-engraving, and have hitherto been considered as insur- 

 mountable, in their very nature. They also attend the common etching pro- 

 cesses of the engravers ; here, too, it is found that the fine lines become un- 



