124 Experimental Philosophy. [Lecture 9- 



The galvanic influence cannot be powerfully 

 excited without a combination of three con- 

 ductors, two of one class and one of another. 

 When two of the three bodies are of the first class 

 (as two metals, zinc and silver, or zinc and copper 

 with water or an acid), the combination is said to 

 be of the first order. But it is an indispensable 

 requisite that one of the three conductors should 

 have a chemical action on one or both the others : 

 thus water, as containing oxygen, has an action 

 on the metals ; if it is impregnated with oxygen 

 gas its action is increased, and much more power- 

 ful than that of water deprived of air by boiling ; 

 and if a small quantity of any of the mineral acids 

 is added, the effect will be still greater. Thus 

 the agitation ore xcitement occasioned by the 

 action of an acid principle is the source of gal- 

 vanism, as the excitement occasioned by friction 

 is of electricity. 



Yet it will appear by an easy experiment that 

 the galvanic influence has a powerful agency in 

 directing and increasing this chemical action. 

 A glass tube (fig. 42.) about 4 inches long has its 

 extremities completely stopped by two corks, A, 

 and B. An oblong piece of zinc, CD, is thrust 

 through one of the corks, and projects within and 

 without the tube. In the other cork is fixed a 

 silver wire projecting with the extremity F, within 

 the tube, while its other extremity is bent so as to 

 come near the projecting part of the zinc C. If 

 then the tube between the corks is filled with 



