218 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 193 5 



when rubbed behaved in one respect quite differently from amber ; a 

 bit of gold leaf excited by contact with the glass tube is then repelled 

 by the glass but attracted by excited amber. "And this ", said Du 

 Fay, " leads me to conclude that there are perhaps two different 

 electricities." These he distinguished accordingly as vitreous and 

 resinous, and laid down the law that like electricities repel each 

 other and unlike attract. 



To explain the same phenomenon Franklin (1747) postulated a 

 single electric fluid of which all bodies were normally full. If a body 

 acquired more than this normal amount he called it plus, or posi- 

 tively electrified, and if its charge was less than normal, minus, or 

 negatively electrified. 



Franklin's hypothesis had simplicity in its favor; it required one 

 less assumption than that of Du Fay. In this respect it obeyed more 

 closely the rule laid down by Newton : 



We are to admit no more causes of natural things, than such as are both 

 true and sufl3cient to explain their appearances * * * for Nature is pleas'd 

 with simplicity and affects not the pomp of superfluous causes.' 



This simplicity of Franklin's hypothesis, added to the reputation 

 which he himself rapidly attained in scientific circles, gave the one- 

 fluid theory an advantage over its competitor for the time being, but 

 a serious theoretical objection was soon raised agamst it. Since on 

 this theory a negative charge meant a deficiency of electric fluid, 

 there must be a limiting value of negative charge, namely, when the 

 body is completely emptied of the electric fluid ; but two such bodies, 

 both being negatively charged, should repel each other — and why ? 



There was much hesitancy on the part of the one -fluid advocates 

 about pushing this argument to its logical conclusion. It remained 

 for a bold German named Aepinus (1759) to seize the bull by the 

 horns and assert that matter devoid of electricity is self-repellent. 



This doctrine came as a shock to a generation many of whom could 

 remember Newton. It was useless to point out that Newton had 

 deduced the law of gravitation by observation of bodies that pos- 

 sessed their normal amount of electricity, and that the behavior of 

 matter with the maximum negative charge was something which no 

 one had ever observed. The one-fluid theory had received a serious 

 jolt from which it never recovered; this argument was used against 

 it as late as the 1830's. The attention of theoretical physicists of the 

 eighteenth century was turned toward the two-fluid theory, and dur- 

 ing the closing years of that century and the early part of the nine- 

 teenth the work of Coulomb, Laplace, Biot, and Poisson produced an 

 elaborate and elegant mathematical theory which so well described 

 all the electrostatic phenomena then known that by 1830 the two- 

 fluid theory was generally accepted. 



* Newton, Princlpia, book 'S : Rules of reasoning in philosophy. 



