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which I am bound to refer to. Professor Birks, in criticising my paper on 
Force and Energy, says : — 
“ Still further, in §§ 24, 25, light and heat are said to be accurately defined as 
‘ a very brisk agitation of the insensible parts of the object.’ Yet in § 29 we 
read that they ‘ have frequently been illogically designated as “ modes ot 
motion ” by able physicists,’ and this ‘ has led them into a hopeless confusion 
of the terms, force, energy, and motion.’ But a very brisk agitation is cer- 
tainly a mode of motion, so that the paper is a fresh instance of that confu- 
sion of which its writer justly complains.” 
Now, I fear Professor Birks has overlooked my argument. I will read one of 
the paragraphs in my paper which is referred to, and leave it to speak for itself. 
I say at the close of the 23rd section of my paper “John Locke writes: — ‘Heat 
is a very brisk agitation of the insensible parts of the object, which produces in 
us that sensation from whence we denominate the object “ hot ” ; so what in 
our own sensation is heat, in the object is nothing but motion.’ It would 
be, perhaps, still more precise to say, ‘ heat arises from,’ &c., in place of 
‘ heat is,’ &c., because the latter part of the definition states heat to be, 
not the motion, but the perception of it.” Then I go on, in my 24th 
section, to say : — “ Precisely the same definition will serve equally for 
light, if ‘ light ’ be substituted for ‘ heat,’ and ‘ luminous ’ for ‘ hot.’ 
It would then read thus : — Light is a very brisk agitation of the in- 
sensible parts of the object which produces in us that sensation 
from whence we denominate the object luminous ; so that what in 
our sensation is light, in the object is nothing but motion.” I there- 
fore maintain, in the last- few lines of my 23rd section, and point- 
edly state that heat and light are not to be accurately defined as a very 
brisk agitation of the insensible parts of the object, but as the result of that 
brisk agitation. To say that one thing is another, and to say that one thing 
is the result of another, are certainly very different statements. What I have 
said will show that he speaks of me as having made the very error which I 
have imputed to others. If the definition of force which I have given be 
taken as the true definition, I think that that, with what I have said in my 
paper, and with the illustrations which I have given to-night, will establish 
the point that the conception of force is a distinct mental conception, apart 
from the conception of its operation. You may have a magnet, and you may 
have a mental conception of the force situated in the pole of that magnet ; 
and that conception is quite independent of any action of that force. You 
may have iron, which the magnet attracts, or bismuth, which it repels, or 
the similar pole of another magnet, which it repels ; but the idea of the force 
existing in the magnet is to me entirely independent of its exercise upon 
another body. So in the same way the conception of the existence of a force 
appears to me totally different from the conception of the action which it 
produces. There is only one other point I want to refer to, and I certainly 
must admit it was an oversight on my part. Professor Birks refers to my 
paper as having spoken of force producing, and being the cause of that action 
between particles or masses of matter by which they are drawn together and 
