REPORT ON ELECTRIC IT Vj MAGNETISM, AND HEAT. 3 
they had both a deficiency of the repulsive element ? zEpinus 
therefore found himself obliged to ascribe a mutual repulsion 
(incomparably greater than the force of gravity) to the particles 
of all bodies. 
Coulomb established, what Mayer and Lambert had already 
ascertained, that the electrical force follows the law of the in¬ 
verse square of the distance; and this was to be taken therefore 
as the law of the attraction and repulsion of the particles of the 
fluid in the iEpinian theory: but the theory stood in need of 
modification, and this it received from Coulomb. 
Coulomb’s reform of the electric theory of ./Epinus consisted 
in assuming two opposite fluids, each attracting the particles of 
the other and repelling its own, by which means the repulsive 
force of the particles of the bodies was no longer admitted. The 
theory of one fluid had had many adherents, and the most persua¬ 
sive argument in its favour was its greater simplicity; but when 
it was shown to involve the assumption that the particles of all 
bodies, though they attract each other with a force varying in¬ 
versely as the square of the distance by the law of gravitation, 
repel each other by a much greater force varying according to 
the same law, the doctrine of a single fluid certainly lost at least 
the prerogative of simplicity. And when further it appeared that 
the same reasoning applied to magnetism ; and that, on the hy¬ 
pothesis of one electric and one magnetic fluid the particles of 
iron must have, besides the attraction of gravitation, two other 
forces of mutual repulsion, one electric and one magnetic, all 
the three forces following the same law, it could not be doubted 
that the superiority of simplicity was transferred to the side of 
the hypothesis of two opposite electric and two opposite mag¬ 
netic fluids. These hypotheses accordingly Coulomb adopted. 
It now became necessary to calculate the results of the hypo¬ 
thesis ; and so far as this went, the results of the iEpinian and 
Coulombian theories were the same. The calculation could be 
performed in a very limited range of cases, according to the 
mathematical methods which were in common use at that time. 
iEpinus had traced the general character of these results in his 
work published in 1759; and Cavendish, in the Phil. Trans . 
for 1771 ? had examined them further, assuming any law between 
the inverse simple power and inverse cube of the distance, but 
obviously inclining to the inverse square. But Coulomb had 
invented and employed delicate methods of ascertaining precisely 
the distribution of electric intensity in many cases not contem¬ 
plated by preceding writers, and had to calculate the results of 
such cases for the sake of that comparison on which his theory 
was to rest. 
