6 
FIFTH REPORT —1S35. 
initiation of the principal systems to which electric phenomena 
have given birth.” 
Coulomb died in 1806, I believe without fulfilling the inten¬ 
tion he here expresses. But we may allowably conjecture, I 
think, that the tenderness with which he here speaks of the 
theory of one fluid was not so much the expression of his own 
conviction as the effect of a wish not to shock the predominant 
persuasion on this subject, which from the time of Franklin had 
been in favour of one fluid only. We can hardly suppose Cou¬ 
lomb to have allowed this theory of one fluid more than a very 
minute comparative probability, not worth reckoning, when we 
recollect that he had so strongly pointed at the absurdity of the 
hypothesis which that theory necessarily involves, of the mutual 
repulsion of all the particles of matter. 
Ever since the time of Newton, it has been customary for per¬ 
sons, attempting to pronounce judgement upon a philosophical 
theory, to refer to his u Rule of Philosophizing,” u Causas rerum 
naturalium non plures admitti debere quam quee et verse sint, 
et earum phsenomenis explicandis sufficiant.” So far as the 
question of one or two fluids is concerned, it is to be recollected 
that we must at any rate have two causes , attraction and repul¬ 
sion ; and that, in fact, to attribute these forces to the fluids 
alone is to take fewer kinds of causation than to attribute them 
both to the particles of the fluids and of the body. Also in such 
estimations much stress has usually been laid upon the condition 
of the “ vera causa”. There is this very material difficulty in 
the application of that part of the rule, that it supposes us al¬ 
ready to be in possession of the means of distinguishing true 
causes from untrue. If we really had such a criterion, by much 
the most important Rule of Philosophizing would be one in 
which the criterion should be stated. But if by a vera causa 
we mean (as it would seem men usually do mean) a cause al¬ 
ready supposed to be known by mechanical effects ; although the 
rule so understood appears to be very arbitrary, we may apply it 
to the theory of electricity. The existence of some electric fluid 
(whether one or two) as a true cause of the phenomena may, on 
this view, be held to be proved by the facts which suggested such 
a conception from the first;—by the accompaniments of the dis¬ 
charge,—the spark,—the sound; and especially the mechanical 
effects,—the shock, the power of striking, breaking and penetra¬ 
ting material objects. And thus the belief of one electric fluid at 
least is forced upon us as a physical truth, while the theory of 
two fluids rather than one is established by its being proved to 
involve, in reality, the simplest system of assumptions under 
which the phenomena can be explained. 
