702 THE A PPL 1C A TIONS OF PH YSIGA L FORCES. [BOOK v. 



genius has made such powerful helps to navigation, have been 

 increased in brilliancy and range of light. 



In order to complete this view of the application of electricity 

 it remains to give some account of those which are based on the 

 chemical effects of currents; that is to say, on the still mysterious 

 phenomena which are generally regarded in science as themselves 

 one of the generators of dynamic electricity. 



Electro-plating, electro-chemistry, are the names under which 

 these applications are generally known ; applications of which science, 

 manufactures, and art have all equally found to be profitable ; one 

 word on their common principle will suffice to justify the distinction 

 we have just made. 



Let us first call to mind the phenomena produced when a galvanic 

 current is made to pass through a saline solution. Take, for example, 

 a solution of sulphate of copper. So soon as the circuit is closed and 

 the current produced, decomposition of the salt takes place ; bubbles 

 of oxygen are disengaged at the positive electrode, and copper is 

 deposited in a metallic state upon the bar which forms the negative 

 electrode. This phenomenon of decomposition was already known to 

 those physicists who had at their disposal only the original voltaic piles ; 

 but on account of the irregularity of the current, and its rapid 

 falling off, the metallic deposit was generally only a pulverulent crust, 

 and useless for industrial purposes. Science, however, took advantage of 

 it, and chemists were thus enabled to isolate and discover metals till 

 then unknown. The invention of constant batteries such as Daniell's 

 modified the phenomenon in a favourable manner. We have had occa- 

 sion above to cite the discovery of the first electromotor namely, that 

 which Jacobi invented to navigate a vessel on the Neva. If that inven- 

 tion had not the success that its author expected, it was the occasion 

 of a more fortunate discovery, whence has certainly arisen the art of 

 electro-plating. Jacobi, who had employed a Daniell's battery for his 

 experiment with the positive pole formed of plates of very pure and 

 very malleable copper, was astonished to see the plates of platinum 

 of the negative electrode covered with a rough deposit, formed of 

 little scales of brittle copper, the inner surface of which faithfully 

 reproduced all the inequalities of the metal on which they had formed. 

 The illustrious physicist repeated the same experiment with vari- 

 ations, and obtained homogeneous metallic deposits, which, instead of 



