»b CHEMISTRY. 



and the copper end positive. When the two extremities are joined, 

 the current continues to pass from the copper to the zinc ; so that in 

 fact, we may consider that there are two currents : one, in the 

 battery, passing from the zinc to the copper ; the other, out of the 

 battery, going from the copper to the zinc, as shown by the arrows 

 in the preceding figure. 



Cruikshank''s trough, is a modification of Volta's pile ; it consists 



of numerous pairs of zinc 

 Fig- 13. and copper plates soldered 



together,and cemented water- 

 tight into a mahogany trough, 

 which thus becomes divided 

 into a series of cells or com- 

 partments, capable of receiv- 

 ing the exciting liquid. This apparatus is well adapted to exhibit 

 effects of tension, and to give shocks. 



The quantity of electricity set free, is to be distinguished from its 

 tension ; the first is measured by its chemical effects — particularly 

 the power of decomposition ; the last, by its power of overcoming 

 obstacles, and passing through imperfect conductors. 



Different theories as to the nature a?id cause of galvanism. — 

 Galvani supposed it to be developed in the animal upon which he was 

 experimenting, and that the metals served merely as conductors ; 

 Volta, on the other hand, supposed it to be developed by the contact 

 of dissimilar metals. Subsequently, Faraday brought forward the 

 theory oi chemical action, which is now most generally adopted. 



The most important effect produced by galvanism, is that of the 

 decomposition of compound bodies. It was by means of this agent 

 that Sir H. Davy made the discovery of the metals of the alkalies. 

 When compound bodies in the fluid slate are traversed by a galvanic 

 circuit, a decomposition usually takes place according to a uniform 

 and constant rule ; certain elements, as oxygen, chlorine, iodine, 

 acids, &c., appearing at the ^positive or copper end of the battery, 

 and others, as hydrogen, the metals, &c., at the zinc or negative 

 end. Hence the division of bodies by chemists according to their 

 electrical habitudes ; those which go to the positive pole being called 

 electro-negative, — at the head of which stands oxygen ; and those 

 which go to the negative pole being called electro-positive, of which, 

 hydrogen is the first. 



The electrodes or poles of a battery are the points of the circuit 

 (usually the extremities) where the electrical phenomena are mani- 

 fested. The decomposition of a fluid by galvanic action is termed 

 electrolysis ; and the liquids which are capable of being thus acted 

 on are named electrolytes. 



Magnetic effects of galvanism. — Among the effects of galvanism, 

 its influence over the magnet is very remarkable. Although the 



