82 



CHEMICAL PHYSICS. 



FIG. 34. 



of very high electromotive force (1000-3000 volts) passes between terminals of 

 pencils of hard carbon, which, while infusible, burn away gradually, requiring 

 an automatically acting contrivance to keep the two pencils at a constant dis- 

 tance. In the incandescent lamp a filament of carbon, fastened in an exhausted 

 glass globe, is heated to a white heat by a current of about 50 to 120 volts. 



Conversion of electrical energy into chemical action. A highly 

 important effect of electric currents is their power to cause chemical 

 decomposition i. e., the splitting- up of matter into two of its com- 

 ponent parts. Thus, if the terminals of a battery are placed in a 

 vessel with acidified water, gas-bubbles rise from 

 both terminals, and on examination the gases 

 are found to be hydrogen and oxygen, which 

 are the constituents of water. In order to col- 

 lect the gases and measure their volume, the 

 apparatus shown in Fig. 34 may be used. It 

 consists of three connected glass tubes, which 

 are filled with water acidified with sulphuric 

 acid. The electric current is made to pass 

 through the liquid from the poles, in this case 

 preferably pieces of platinum foil fastened to 

 platinum wire fused in the glass tubes. On 

 passing a current through the liquid, oxygen 

 rises from the positive, and hydrogen from the 

 negative pole. The process of splitting up a 

 compound body by electricity is called elec- 

 trolysis ; the bodies undergoing decomposition 

 are termed electrolytes. The metallic conductors 

 by which the current enters and leaves a liquid 

 or gaseous electrolyte are called poles or elec- 

 trodes, and are designated by the same names 

 given to the plates in the generating cell i. e., 

 they are called positive ( + ) pole or anode, and negative ( ) pole 

 or cathode. The decomposition product appearing at the positive 

 pole is said to be electronegative, the one appearing at the nega- 

 tive pole is electropositive. 



Electrolysis of water. 



Electropositive are : 

 Hydrogen, 

 Metals, 

 Bases and basic radicals. 



Electronegative are : 

 Oxygen, 



Halogens (chlorine, etc.), 

 Acids and acid radicals. 



When an electric current passes through a solution of any salt, 

 this is split up into the base and the acid composing the salt. If 



