200 ELEMENTARY GENERAL SCIENCE CHAP. 



the chemical elements, and that it is so regarded because it can 

 neither be decomposed by the passage of an electric current nor 

 by any other known methods. The great deflection of the 

 needle of the galvanoscope reveals the fact that a considerable 

 current passes through its coil of wire, hence we say that mercury 

 is a good conductor of the electric current, or expressing the 

 same truth in other words, that it offers very little resistance to 

 the flow of the current. 



Similarly, it would be found that other metals, when in the 

 liquid condition, which they can assume if the temperature is 

 raised sufficiently, are also good conductors of the electric 

 current. 



2nd Case. Passage of the Current through Turpentine. There 

 is in this case 110 deflection of the needle of the galvanoscope ; 

 it is therefore evident that no current passes through the coil 

 of wire round the needle, and since the battery is arranged 

 precisely as in the previous experiment with the mercury, the 

 explanation must be that the turpentine prevents the flow of 

 the electric current round the circuit. Turpentine is conse- 

 quently known as a non-conductor, a class of bodies which also 

 includes such liquids as petroleum and other oils. 



3rd Case. Passage of the Current through Acidulated Water. 

 Here we have the current conducted and the liquid decomposed 

 by the passage of the current. This is the condition of things 

 in all liquid compounds which conduct the electric current. 

 Such a decomposition as this is known as electrolysis, and we 

 shall have to study this case more fully. 



Electrolysis of Water. Pure water is a very bad conductor 

 of the electric current, and hence, as in Expt. 245, it is necessary 

 to add a drop or two of acid to make it conduct. To understand 

 exactly the result of the passage of the electric current, some 

 means must be devised by which the gases which appear at the 

 platinum plates can be collected. Such an arrangement con- 

 stitutes what is known as a voltameter. [This piece of apparatus 

 will be again referred to later on (Chap. XVI).] A convenient 

 pattern to use for the decomposition of water consists (Fig. 106) of 

 a glass vessel in the bottom of which are fixed two slips of 

 platinum connected, by means of copper wires, with two binding 

 screws. Before connecting the binding screws with the poles of a 

 battery, acidulated water is poured into the vessel and two glass 

 tubes, of exactly equal size and carefully divided into equal 

 volumes by divisions etched on the glass, are completely filled 



