154 



SMITH'S INTERMEDIATE CHEMISTRY 



the battery, or other source of the current must be kept going or 

 the chemical change stops. The inverse of this is likewise familiar. 

 If we place in dilute sulphuric acid a stick 

 of the metal zinc, we find that hydrogen is 

 given off (Fig. 54), that the zinc goes into 

 solution as zinc sulphate (p. 102), and that 

 a large amount of heat is developed. The 

 presence of small quantities of other metals 

 in the zinc catalyzes this reaction. If im- 

 pure zinc in fine particles, with much sur- 

 face, is used, the temperature of the liquid 

 may even rise spontaneously to the boiling- 

 point. This form of the action produces 

 heat. If, however, we attach the same 

 stick of zinc to a copper wire and, having 

 provided a plate of platinum also con- 

 FIG. 54 nected with a wire, immerse the two simul- 



taneously in the acid (Fig. 55), then a 

 galvanometer, with which the wires are connected, shows at once 



FIG. 55 



the passage of a current of electricity round the circuit. Exactly 

 the same chemical change goes on as before. The sole difference 



