1870.] Physics. 571 



the figure appears at once; if cold, a latent image exists, which 

 may be developed by heat. The non-eonducting surface is prepared 

 by coating a plate of metal with an even film of pitch. Pieces of 

 sheet tin, 3 inches square, coated with films of pitch of a thickness 

 varying between 0"01 and 0'02 inch were used; the pitch was 

 the ordinary article of commerce freed from sand, &c, by being 

 melted and strained through a muslin bag. The author gives cuts 

 of the figures as produced by frictional electricity and the induction 

 coil. 



Metallic iron, as obtained by the electric current, has been ex- 

 amined by C. Collas. He employs a weak solution of chloride of 

 iron, which is decomposed by the aid of a Bunsen battery ; perfectly 

 pure iron is thus obtained, which is very friable, highly oxidizable, 

 especially when moisture is present. When this iron in the state 

 of fine powder is poured in a bottle when the atmosphere is very 

 moist, the iron is instantaneously oxidized, water decomposed, and 

 the evolution of hydrogen causes the bursting of the bottle. 



A new method of copper extraction and its separation from other 

 metals is published by Mr. J. Elkington. The principle consists 

 in applying electricity for dissolving the copper contained in the 

 crude metal obtained by the usual smelting methods, and for de- 

 positing that metal galvanically upon plates of copper, causing the 

 other foreign metals to fall to the bottom of the vessels in which 

 the operations take place ; copper containing very small quantities 

 of silver may be advantageously treated thus for the recovery of 

 the last-named metal. 



An improvement in galvanic batteries has been devised by Mr. 

 W. Poole Levison, of Cambridge, Mass. While making use of a 

 small bichromate of potash battery he discovered that the addition 

 of nitric acid to the mixture of potassic bichromate and sulphuric 

 acid contained in its porous cups, conferred upon it the virtue of 

 steadiness without involving the evolution of annoying fumes. For 

 over two months during last summer the author had in almost 

 constant action a combination of twenty-three large Bunsen cells 

 charged with dilute sulphuric acid and the triple mixture mentioned, 

 and " set up " openly upon the floor of the room. Not only did he 

 work about it with perfect comfort, but left choice brass instruments 

 in its immediate neighbourhood with impunity. Its energy never 

 fluctuated, but after remaining for some time steady declined, pre- 

 cisely as if the electro-negative plates were bathed in nitric acid 

 only. To a cooled mixture of potassic bichromate solution and 

 sulphuric acid (perhaps preferably in atomic proportions) add nitric 

 acid. The proportion of nitric acid may be greatly varied, as its 

 office is merely to transfer oxygen. 



A research on the best methods of tinning of iron without the 



VOL. VII. 2 Q 



