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is produced. We must hold that it is nothing short of folly 
to reason as if that which agitates a body were the agitation 
which the agitating agent or force produces. Men apologize 
for such reasoning by saying that they cannot find suitable 
language in which to convey to the ordinary mind the new 
ideas which they have found; but the apology is worth ess. 
Let them only have ideas that can be clearly thought, and they 
will soon get the right words by which those ideas can be 
correctly expressed. It is not the words “force” and ‘ motion 
with which we have any reason to find fault, hut the idea ol 
that which produces motion being motion itself. _ Heat is 
motion and nothing else; ” it is a state of matter m motion, 
and nothing more ; the thing heat is unknown; yet this very 
heat is the force which produces this very motion that is, 
heat produces itself! Not that some heat produces more heat, 
but that one heat produces that very heat ! ! The words are 
only too good, for they make the absurdity of the idea per- 
feC But P the e reis invaluable instruction on this very subject to be 
gathered from Grove’s teaching in his admirable essay. He 
gives as an illustration of the correlation of forces, a chain ot 
changes, each link of which is only a peculiar mode of motion. 
He says — “ At my lectures in 1843 I showed an experiment by 
which the production of all the other modes of force by light 
are exhibited. I may here shortly describe it A prepared 
Daguerrotype plate is enclosed m a box filled with water, 
having a glass shutter over it. Between this glass and the 
plate is a gridiron of silver wire ; the plate is connected with 
one extremity of a galvanometer coil, and the gridiron of wire 
with one extremity of a Brequet’s helix — an elegant instrument 
formed by a coil of two metals, the unequal expansion o 
which indicates slight changes of temperature— the other 
extremities of the galvanometer and helix are connected with 
a wire and the needles brought to zero. As soon as a beam ot 
either daylight or oxyhydron light is, by raising the s ^ utte J 
permitted to impinge upon the plate, the needles are deflected. 
Thus light being the initiatory force, we get chemical action 
on the plate, electricity circulating through the wires, magnet- 
ism in the coil, heat in the helix, and motion in the needles^ 
He speaks of these successive changes m the state ot the 
matter in hand “ as modes of force,” when all his reasoning 
goes to show that they are modes of motion, and, as he says 
in words already quoted, “nothing else.” He speaks of hglit 
as the initiatory force, though he proves elsewhere that light 
is not a force at all, but a state of motion or agitation m the 
molecules of illuminated matter. But it is not with this that 
