1866.] 



Indications of Animal Electricity. 



159 



— and so also in similar cases, the movement of increased divergence of the 

 gold leaves in both electroscopes arising from the reaction of the charge in 

 the instrument being added to or subtracted from the movement of the 

 gold leaves produced by the action of free electricity, according as these 

 movements happen to be in the same or in opposite directions. 



These, then, being the facts, it is easy to see how, by using two 

 electroscopes, it is possible to ehminate the reaction of the charge of 

 electricity in the instruments upon the gold leaves, and to make this 

 reaction tell in making more obvious the action of very minute quantities 

 of electricity. It is easy to eliminate the reaction of the charge of electricity 

 in the electroscopes upon the divergence of the gold leaves ; for this reaction 

 causes slightly increased divergence of these leaves in both instruments. 

 It is easy to make the reaction of the charge of electricity in the electro- 

 scopes upon the divergence of the gold leaves tell in making more obvious 

 the action of free electricity upon the divergence of these leaves ; for it is 

 plain that in the electroscope in which the action of positive electricity- 

 causes increased divergence of the gold leaves, this movement will be aided 

 bv the increased divergence of these leaves arising from the reaction of the 

 charge of electricity in the electroscopes, and, vice versd, that in the 

 electroscope in which negative electricity causes increased divergence of 

 the gold leaves this movement will be aided by the increased divergence 

 of these leaves arising from the reaction of the electricity in the instrument. 

 Nor does it follow that one of the electroscopes is in reality superfluous. 

 A priori, indeed, it might be supposed that one electroscope would be 

 sufficient. It might appear enough to take the movements of increased 

 divergence of the gold leaves as evidence of the action of one kind of 

 electricity, and the movements of diminished divergence as evidence of the 

 other kind of electricity. But when dealing with minute quantities of 

 electricity, it is found practically that the movements of diminished 

 divergence are not so easily produced as those of increased divergence, 

 and that there are impediments to free movement in this direction, arising 

 from the clashing of the movement of diminished divergence due to the 

 action of free electricity with the movement of increased divergence due to 

 the reaction of the charge of electricity in the electroscope. In short, 

 the plain truth appears to be, not only that the two electroscopes act as a 

 check upon each other, and show the same thing from two different points 

 of view, but that they furnish evidence which in itself is far more con- 

 clusive, when dealing with minute quantities of electricity, than can be 

 got from either instrument singly. 



In the description of the experiments upon which I am now about to 

 enter, it is necessary to be able to distinguish the two electroscopes the 

 one from the other ; and I propose, therefore, to speak of the instrument 

 in which the gold leaves are charged with positive electricity, and in which 

 positive electricity causes increased divergence of these leaves, as the Positive 

 Electroscope, and of the instrument in which the gold leaves are charged 



