448 M. Van Marum on [Dec. 



been brought forward in favour of the doctrine, and then to sub- 

 jnit to your judgment whether the observations adduced against 

 Frankhn's system are so well founded, and of such a nature, that,, 

 notwithstanding what I have now said respecting it, greater 

 probability is to be attached to the contrary system ; and in this: 

 I have attempted to foUow the course by which the old opinion 

 has been endeavoured to be re-estabhshed in France. 



In 1787, Haliy pubhshed the theory of Franklin as it had been, 

 developed by M. CEpinus, Member of the Academy of St. 

 Petersburgh, in his Tentamen Theoriee Electricitatis et Mag- 

 netismi. Peters. 1759 ; and this he drew up in such a manner as 

 by excluding as much as possible all mathematical calculations 

 and propositions, to render it intelligible to a great number of 

 readers : this he entitled " Exposition Raisonnee de la Theorie 

 de I'Electricite et du Magnetisme d'apres les Principes de M. 

 Qjlpinus. In the preliminary discourse of this work, he ob- 

 served, respecting CEpinus's theory, " it must be allowed that 

 among the principles upon which the theory of M. CEpinus is 

 founded, there is one so greatly at variance with sound physical 

 principles that the author himself long hesitated whether he 

 should admit it, nor was it until after mature examination that 

 he determined upon doing so. The principle in question is that 

 the particular atoms of bodies possess a mutual repulsive power, 

 as do likewise the atoms of the electric or magnetic fluid. 

 M. CEpinus proves that the existence of this force is a necessary 

 consequence of that of two forces, upon which is founded the 

 theory of Franklin. In fact, it is not more inconsistent to admit 

 of a repulsive force between the atoms of bodies than betweea 

 those of fluids, whether it be electric or magnetic, because the 

 term force expresses merely a fact of which we do not inquire 

 into the cause. The more especial point of objection against 

 similar forces is, that they cannot be reconciled with the princi- 

 ple of universal gravitation. But as the forces in question do 

 not exert themselves, except in the particular instance of electric 

 or magnetic phenomena ; and since this action is, as it were, 

 suspended by opposing forces when the bodies meet in their 

 natural state, as will be shown in the course of this work, gravi- 

 tation is not thereby rendered less a general force capable o£ 

 proving only local and transient shocks occasioned by the phe- 

 nomena of electricity and magnetism." Yet although in the 

 preceding quotation M. Haliy has endeavoured, by the argu- 

 ments he brings forward, to remove the obvious difficulty which 

 arises, if we admit the principle of a repulsive force existing 

 between the atoms of bodies, it appears that he subsequently 

 considered his own reasonings as insufficient ; for in his Traite 

 Elementaire de Physique, Paris, 1806, torn. i. p. 398, we meet 

 with the following passage : CEpinus was led by his theory to the 

 singular inference that taking into view electric phenomena, the 

 atoms of bodies repel each other. In perusing his work one may 



