1874.] Notices of Books. 525 
book which has long been before the public, we will indicate here 
those passages which are striking from their originality or pre- 
cision, or from some other cause. As to causation, our author 
remarks that the view entertained of it is usually that of Hume, 
who refers it to ‘invariable antecedence,” a cause being that 
which invariably precedes, and in effect that which invariably 
succeeds; and he shows that this definition will not bear strict 
scrutiny. As to the aims of physical science, he remarks— 
“Instead of regarding the proper object of physical science as a 
search after essential causes, I believe it ought to be, and must 
be, a search after facts and relations; that although the word 
Cause may be used in a secondary and concrete sense, as 
meaning antecedent forces, yet in an abstract sense it is totally 
inapplicable.” 
Form is defined as “that active principle inseparable from 
matter which is supposed to induce its various changes,’—not, 
to our mind, a satisfactory definition, although we know not 
where to look for better. Referring to “force in abeyance,” or 
tension, or what we should now call ‘ potential energy,” Sir W. 
Grove gives us the following comprehensive and perfectly logical 
remarks :—‘‘ But it may be objected, if tension or static force be 
thus motion in abeyance, there is at all times a large amount of 
dynamical action subtracted from the universe. Every stone 
raised and left upon a hill, every spring that is bent and has re- 
quired force to upraise and bend it, has for a time, and possibly 
for ever, withdrawn this force, and annihilated it. Not so; when 
we raise a weight and leave it at the point to which it has been 
elevated, we have changed the centre of gravity of the earth, 
and consequently the earth’s position with reference to the sun, 
planets, and stars; the effort we have made pervades and shakes 
the universe; nor can we present to the mind any exercise of 
force which is not thus permanent in its dynamical effects. If, 
instead of one weight being raised, we raise two weights, each 
placed at points of the earth diametrically opposite to each other, 
it would be said, here we have compensation, a balance, no 
change in the centre of gravity of the earth; but we have in- 
creased the mean diameter of the earth, and a perturbation of 
our planet, and of all other celestial bodies, necessarily ensues.” 
The definition of heat as ‘“‘a communicable molecular repulsive 
force”? would scarcely be very generally accepted, we think. A 
repulsive force cannot readily be defined, or in the abstract be 
conceived ; but if we regard heat as vibratory motion, we can at 
once realise how expansions are produced by the addition of such 
motion to congeries of molecules, which vibratory motion will 
of necessity drive them further apart. 
Of electricity, Sir W. Grove says—‘“I think I shall not be un- 
supported by many who have attentively studied electrical 
phenomena, in viewing them as resulting not from the action of 
a fluid or fluids, but as a molecular polarisation of ordinary 
VOL. IV. (N.S.) ax 
