human knowledge. One observes that which actually occurs, 
but he depends upon another, perhaps upon several others, 
for the true interpretation of that which he has observed. 
One of the best essays on what is called “ force ” in nature 
is by Grove. He styles his subject the “Correlation of Forces, 33 
and shows, I think, with great conclusiveness of argument, 
that all the “ affections 33 of matter (as he calls them) are only 
modes of that state of material substances which we usually 
call motion. What all understand as motion is (as we all 
know) a state of the moving object generally as a mass ; but 
heat is the same and light also ; only these are motions of the 
molecules or constituents of the mass. Electricity, magnetism, 
and “ chemical affinity,” or, as I should prefer to call it, 
chemical action, are only modes of agitation or motion in the 
matter affected. All the so-called “ imponderables,” such as 
“ caloric,” the electric “ fluid,” and the “ ether,” imagined as 
filling up the spaces between the atoms of matter, are thus 
disposed of as nonentities. 
Grove speaks of the “inertia 33 of matter, but we are not 
sure what he means by the word, for he speaks of all matter 
as in a state of perpetual change. According to one mode or 
another every atom of the universe is regarded by him as in 
continual motion. He speaks of portions of matter as held in 
‘ f equilibrium ” by equal and opposing forces, and in this alone 
can we see anything that can be called inertia according to his 
view. So far, however, we find no great reason for adverse 
comment on his ideas. But when we endeavour to get hold 
of his idea of “ force ” he escapes us effectually. He says — 
“ The dynamic theory regards heat as motion, and nothing 
else.” * This is his own theory. So he regards heat as 
motion and nothing else.” But he says a little further on 
(page 69) that “We only know certain changes of matter, 
for which changes heat is a generic name ; the thing heat is 
unknown.” Then he goes on to say that “ heat having been 
shown to be a force capable of producing motion, and motion 
to be capable of producing other modes of force, it necessarily 
follows that heat is capable, mediately, of producing them.” 
We quote these words for the purpose chiefly of showing how 
loosely even such eminent men will reason. If heat is 
‘ f motion and nothing else,” how can it be “ force ? ” Or, if 
heat is force capable of producing motion, how can it be that 
very motion and nothing else ? Motion is a state of matter — 
force is not , as Grove’s own words seem to show, but a pro- 
perty or an energy, by which, as he says, this state of motion 
* Grove’s Correlation of Physical Forces , page 66, edition 1862. 
