3 28 HISTORY OF SCIENCE. 



It was afterwards pointed out that to make this hypothesis agree with 

 the fact, we must assume that there exists also a repulsive action 

 between particles of matter at considerable distances ; or if this be 

 held inadmissible, as indeed it is, there are certain other purely hypo- 

 thetical assumptions required to reconcile the theory with facts, and 

 such additional assumptions deprive it of the advantage of its original 

 simplicity. Franklin's theory had the prestige of accounting- in an 

 intelligent way for the phenomena of the Leyden Phial, which were 

 at this time astonishing everybody and perplexing the ablest philo- 

 sophers in Europe. " The electric fluid," said he, " spreading over 

 the interior surface of the bottle, repels the fluid which is spread over 

 the external covering ; so that while an excess of fluid is accumulated 

 in the interior, a corresponding deficiency is maintained on the ex- 

 terior.'"' He showed that when a Leyden phial is placed on an insu- 

 lated stand, it cannot be charged unless the outer coating has some 

 conducting body sufficiently near it, when, for every spark passing 

 between the machine and the inside, a spark will pass between the 

 outside and the conducting body near it. A series of Leyden jars 

 may be simultaneously charged by connecting the inside of each with 

 the outside of the preceding one, all being kept insulated except the 

 last in the series. It need scarcely be said that Franklin regarded the 

 effects of the Leyden jar shock, spark, etc., as due to the passage out- 

 ward of the excess of electricity accumulated in the inside when that 

 vra& positively dectrifitd ; and to the rush inwards in the opposite case. 

 Some points may here be mentioned with regard to the theories of 

 electricity, upon which great confusion exists in the minds of unscien- 

 tific readers. We have seen that Du Fay recognized two electricities, 

 or, if you will, two different electric fluids. He called one vitreous, be- 

 cause it is excited on glass by friction ; the other resinous, because 

 friction develops it in resins. These names at first appeared suffi- 

 ciently correct as a description. But at a later period it was found 

 that by friction between two bodies of the same kind, both electricities 

 were developed, and that under some circumstances glass gives the 

 resinous electricity. Then some other terms of distinction appeared 

 desirable, and the words positive and negative were substituted for 

 vitreous and resinous respectively. The reader must here carefully 

 guard against a misconception : positive must not be taken to mean 

 something actually present, and negative the absence of that thing. 

 The words were applied to electricity in the first instance as the ma- 

 thematical distinction of quantities. An engineer speaks of the up and 

 the down line on a railway, without meaning thereby that the sets of 

 rails are inclined in different directions ; so that it is, in fact, merely 

 a common agreement by which one of these terms shall be appro- 

 priated to each line of rails. So likewise the application of the terms 

 positive and negative, as applied to the two kinds of electricities, might 

 be reversed without any change in their meaning, provided everybody 



