138 ANNUAL OF SCIENTIFIC DISCOVERT. 



into S, "which appears at the negative pole, and oxygen at the positive pole. 

 It depends on the concentration of the acid whether the extra decomposition 

 of water accompanies the foregoing products. 



That an electrolyte differing from the simply binary constitution i? capable 

 of direct decomposition by the current, is thus shown in the case of SO", and 

 even with less room for doubt, in the case of anhydrous chromic acid, and 

 chromate of potash, as the researches of Prof. Magnus prove. Liebiy and 

 Kopp's Annual, 18.37. 



ELECTRO-ZINC DEPOSITS ON ENGRAVED COPPER PLATES. 



Bradbury's (of London) process for surfacing engraved copper plates 

 with a coating of pure zinc, by electro-metallurgical means, for the purpose 

 of protecting such plates from wear while printing, is described by the in- 

 ventor as follows : 



To obtain a deposit of pure zinc, capable of printing from fifteen hundred 

 to two thousand impressions or more, before requiring to be removed and 

 renewed, I have recourse to a combined solution of chloride and cyanide of 

 zinc, prepared as follows : 



Chloride of Zinc Solution. In a suitable vessel, dissolve one part chloride 

 of ammonium in eight parts water; place in this a porous cell containing 

 the same solution and a copper plate, which attach to the zinc of a Smee's 

 battery, and in the outer cell place a plate of spelter, which attach to the 

 silver of the above battery for forty-eight hours. 



Cyanide of Zinc Solution. Dissolve one-half pound of cyanide of potas- 

 sum in twelve parts of water; then add as much chloride of zinc as the solu- 

 tion will take up. 



Mix these solutions together in equal parts; use a zinc positive pole and 

 one of Smee's compound batteries, intensity arrangement, charged with one 

 part of sulphuric acid to twelve of water. In from forty-five minutes to an 

 hour, a deposit of the most beautiful lustre will be obtained, capable of 

 yielding from fifteen hundred to two thousand impressions, and even more, 

 according to the experience of the manipulator. 



Mr. Bradbury, however, states in addition, that the durability of engraved 

 copper plates is best increased by covering them with a thin electro-deposit 

 of nickel. " This metal," he says, " gives a surface kinder, for printing pur- 

 poses, than either steel, copper, or any of the known metals ; the reason 

 being simply that, in addition to hardness, it possesses the smoothest, firm- 

 est, and brightest surface to be obtained from electro-deposition. An en- 

 graved copper plate may be covered and recovered ad infinitum, thereby 

 preserving the integrity of the original work to an illimitable number of 

 impressions. 



"Again, if colored inks made from metals be used, such inks do not in 

 the least degree act upon nickel, as they are known to do upon steel and 

 copper. Nickel may be deposited at the same nominal cost as platinum and 

 palladium, namely, from a penny to twopence per square inch. 



"The purity and extreme fitness of nickel deposit, its non-oxidation, 

 the facility of throwing it down, its yielding five thousand impressions and 

 upward from the coating, place the electro-nickel facing immeasurably 

 above electro-iron facing, as it has hitherto been done." 



