644 SCIENCE PROGRESS 



to be so pure that it was nearly unacted on by sulphuric or 

 hydrochloric acid." 



Such is the experience of every one who has had occasion 

 to purify zinc. It is only logical to assume therefore that zinc 

 pure and simple would not be attacked by such acids. 



It may be desirable to insist here that a pure material must 

 ever remain an abstraction, taking into account the fact that any 

 and every substance is necessarily in some degree subject during 

 its preparation to contamination by its environment. 



It has long been known that even highly impure zinc may be 

 " protected " by amalgamation with mercury, that is to say, by 

 rubbing this metal over the surface of the zinc after it has been 

 a short time exposed to the action of acid. The property is one 

 of extreme value and is shared v/ith zinc by no other metal 

 except the allied element cadmium — hence it is that amalgamated 

 zinc was so long made use of in voltaic batteries instead of the 

 cheaper iron, which approaches zinc in electromotive efficiency. 



Faraday explained the effect of impurities in the zinc in 

 causing it to dissolve in acid in the following lucid terms {Experi- 

 mental Researches in Electricity^ 1834, Series VIII., § 998). 



"The cause is, that when ordinary zinc is acted upon by 

 dilute sulphuric acid, portions of copper, lead, cadmium or other 

 metals which it may contain are set free upon its surface; and 

 these, being in contact with the zinc, form small but very active 

 voltaic circles, which cause great destruction of the zinc and 

 evolution of hydrogen, apparently upon the zinc surface but 

 really upon the surface of these incidental metals. In the same 

 proportion as they serve to discharge or convey the electricity 

 back to the zinc do they diminish its power of producing an 

 electric current which shall extend to a greater distance across 

 the acid and be discharged only through the copper or platina 

 plate which is associated with it for the purpose of forming a 

 voltaic apparatus." 



The explanation Faraday gave of the effect of amalgamation 

 is as follows {ibid. § 1,000): 



" It is probable that the mercury acts by bringing the surface, 

 in consequence of its fluidity, into one uniform condition and 

 preventing those differences in character between one spot and 

 another which are necessary for the formation of the minute 

 voltaic circuits referred to. If any difference does exist at the 

 first moment, with regard to the proportion of zinc and mercury, 

 at one spot on the surface as compared with another, that spot 



