INTRODUCTION 15 



First, those which conduct the current when in a pure state and, second, 

 those which conduct the current as a result of the presence of other sub- 

 stances. This latter class of conductors is embraced within the term 

 electrolytic solutions. 



a. Electrolytes Which Conduct in the Pure State. Within this class 

 is included, in the first place, the fused salts. With a few exceptions, 

 the fused salts are excellent conductors of the electric current. Their 

 specific conductance near the melting point being of the order of 1.0, 

 their conductance, therefore, is about 1 X 10" 5 that of silver. The salts 

 are compounds between a strongly electronegative and a strongly electro- 

 positive constituent, and it is seldom that such substances do not possess 

 the power of conducting the current in a marked degree. As the electro- 

 positive or electronegative nature of one or the other of the constituents 

 becomes less pronounced, however, the conductance of the resulting 

 compound is diminished. This is the case, for example, with mercuric 

 chloride. 



When hydrogen is combined with a strongly electronegative element 

 or group of elements, the resulting compound, as a rule, exhibits electro- 

 lytic properties. This, for example, is the case with water, which has 

 been shown to conduct the current slightly when in a pure state. At 

 18 its specific conductance has a value of 0.042 X 10~ 6 . Other com- 

 pounds of hydrogen exhibit similar properties. 



When hydrogen is combined with elements which are less strongly 

 electronegative, the resulting compounds exhibit a lower conducting 

 power. In the case of the hydrocarbons the conductance reaches ex- 

 tremely low values and it is possible that these substances in the pure 

 state do not possess the power of conducting the current. 



While substances in the fused state are, as a rule, better conductors 

 than in the solid state, electrolytic conductors are not restricted to the 

 fused state, since certain substances in the solid state have been found 

 to conduct the current quite as readily as the fused salts. 



b. Electrolytic Solutions. The most common electrical conductors 

 are those in which the conductance is due to a mixture of two or more 

 substances. As a rule, one of these, the solvent, is present in consider- 

 able excess and may itself be only a very poor conductor. In this case, 

 the conductance is said to be due to the addition of the second compo- 

 nent, termed the electrolyte. To this class belong all the ordinary solu- 

 tions of salts in water. In some cases an electrolytic solution results 

 when a substance, which itself in the pure state is a poor conductor, is 

 added to a second substance which likewise is a poor conductor in the 

 pure state. As an example, we may cite solutions of the acids in water. 



