94 THE REALITIES OF MODERN SCIENCE 



pith balls electrified by contact with the rod are found to 

 repel each other. On the other hand, a rod of sealing 

 wax, which has been electrified by rubbing it with cat's 

 fur, will produce effects similar to those of the glass 

 rod. But there is an important difference. A pith ball 

 charged by the glass rod and one charged by the sealing 

 wax will attract each other. These phenomena may 

 be summarized in a simple law, namely : like electricities 

 repel and unlike attract. To the electricity of the glass 

 rod Franklin gave the name " positive" and to that 

 of the sealing wax, the name " negative." However 

 charges of electricity are produced to-day, we still use 

 these names. 



Before returning to the question of atomic composi- 

 tion we shall need to describe further the phenomenon 

 of electrification. It will be found by test that the silk 

 is negatively charged, when the glass rod is positively 

 charged. Furthermore, until the two dissimilar sub- 

 stances are brought into intimate contact, as by rub- 

 bing, they are not electrified. The normal condition of 

 matter, in other words, gives no evidence of electricity. 

 This may be tested by presenting the silk and the glass 

 together to a pith ball, which has been charged, and 

 noting that the latter is neither repelled nor attracted. 

 After the two substances are electrified, if they are held 

 close together and tested by a charged pith ball, they 

 will produce no effect, because their positive and neg- 

 ative electricities neutralize in their effects. Because 

 of this equal effect we conclude that unlike electricities 

 always appear in equal amounts. 



We notice also from such an experiment that when a 

 body gives no evidence of electrification, it may never- 



