690 transactions of the american institute. 



Engraving by Etching. 



Mr. Bai-tlett directed attention to the invention of E. Vial of Paris, wliich 

 remedies the evils formerly attending all etching by means of acids. The 

 principal diflSculty in the old etching process has been, that as the acid dis- 

 solved its way downward into the wax which covered the steel plate, it 

 would also work sideways under the wax, thus widening the channels, as 

 well as deepening them. To prevent this action, M. Vial plunges the steel 

 plate, after a drawing has been made upon it with a greasy ink, into a sat- 

 urated solution of sulphate of copper containing ten percent of nitric acid. 

 By the action of the steel, the copper is reduced from the sulphate, and all 

 portions of the steel plate, not protected, are instantly covered with a coat- 

 ing of metallic copper which protects the steel from further action of the 

 nitric acid. The acid soaks away the ink, and forms channels in the steel 

 beneath, but it is followed by the copper solution, which deposits in these 

 channels a coating of copper, thus protecting the plate from further actioii 

 of the acid. This deposit is made first on the edges of the lines, thus the 

 sides of the channel are first protected. As the acid works longest in the 

 middle of the line, the channel is narrowest at the bottom, that is, it has the 

 V form desired by the engraver. Thus all lines, whether fine or coarse, are 

 made in one operation. 



Old engravings may be reproduced by this process, by transferring the 

 picture to the steel plate, or the design may be first drawn upon paper and 

 then transferred. 



Correlation of Mechanical and Chemical Forces. 



The Bakerian lecture before the British Koyal Society, on this subject, 

 by Henry C. Sorby, contains an account of some interesting experiments 

 made by him, relating to the effect of pressure on the solubilitj' of salts, 

 Messrs. Bunsen and Hopkins, had before proved that substances which 

 expand when fused, have their point of fusion raised by mechanical pressure. 

 Prof. W. Thompson, had shown on the other hand, that water, which 

 expands in freezing, has its point of fusion lowered by pressure; for in this 

 instance mechanical force must be overcome in crystallizations, and under 

 increased pressure, the force of crystalline polarity must be increased by 

 reducing the temperature. Similar principles hold true with respect to the 

 solubility of salt in water. It, when they dissolve, the total bulk increases, 

 pressure reduces their solubility. On the contrarj^, if the bulk decreases, 

 pressure makes them more soluble. The solubility of a salt in water, 

 appears to result from a kind of afiinity, which decreases in force as the 

 amount of salt in solution increases. This affinity is opposed to the crys- 

 talline polarity of the salt, and when the two forces are equal the solution. 

 is exactly saturated. Temperature alters this equilibrium, and the direc- 

 tion of the alteration depends on the mechanical relations of the salt in 

 dissolving. Mr, Sorby makes the provisional conclusion that increased 

 solubility due to pressure varies directly with the change of volume, and 

 inversely with the mechanical equivalent of the force of crystalline polarity. 

 This correlation of forces seemed to him to explain certain facts connected 

 with metamorphic rocks, and the phenomena of slaty cleavage. It is also 



