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to Mr. Moore’s ideas about throwing the imponderables overboard, and here 
I must say that he seems to have failed to represent my views accurately. 
The imponderables which have been thrown overboard are the supposed 
material atoms which constitute light and heat. As I have already explained 
the views once entertained were that there were material particles or atoms 
projected from the hot or luminous body ; but the undulatory theory declares 
that light and heat consist in the perception of certain kinds of vibratory 
•motion. 
Dr. Irons. — The vibratory motion of the ether ? 
Mr. Brooke. — That is another question. It is not necessary to assume the 
existence of ether interstitially deposited in all kinds of matter to convey 
impressions of light or heat : the particles themselves will do it. 
The Chairman. — Mr. Brooke has distinctly stated in his paper his belief 
that ether does not pervade ordinary matter. 
Mr. Brooke. — I have stated that there must be some material medium 
pervading infinite space by which the vibrations constituting light and heat 
are conveyed from the centres of systems to their surrounding satellites ; 
but we are ignorant as to what that is. This medium has been termed 
“ aether,” but what its nature may be I do not pretend to say ; I only take 
it to be matter of some kind in an exceedingly attenuated condition. The 
term “jelly-like” which has been applied to it has been taken up sarcas- 
tically by some, but it merely means this, that the mechanical properties of 
the ether more resemble the mechanical properties of a jelly than those of a 
gas. It means nothing more than that gas and air- have certain mechanical 
properties, while gelatinous substances have certain other mechanical proper- 
ties, and that the mechanical properties of ether more resemble the 
mechanical properties of a jelly than the mechanical properties of a gas : — 
there is nothing more meant than that. Now with regard to the potential 
energy, or “ energy of position,” in the stone to which Mr. Moore objected : 
a stone put up upon a shelf has a potential energy which a stone upon 
the ground has not. Let them both fall down the mine, then the 
one dropped from the shelf above will fall with greater velocity than 
the other. It has acquired a power which enables it to fall with greater 
velocity than the other. That is the simple meaning of potential energy — 
the energy which the stone acquires in being raised from the ground. As to 
the “ conservation of power ” I cannot say anything. Before I can deal with 
that, I must ask you to define power, and when I know the definition I will 
say whether the “ conservation of power ” is the same thing as the “ conser- 
vation of energy.” 
Mr. Moore. — May I say one word ? Power cannot be defined. The 
truest definition is “ power is power,” and that is all ; but every one knows 
what it is. Before I put forward a volition, I am conscious that I have the 
power to do it. But it does not admit of definition. 
Mr. Brooke. — I take it, then, from Mr. Moore’s owm lips that “ power ” is 
indefinable. Then the “conservation of power” means the conservation of some- 
thing indefinable, but the “conservation of energy” means the conservation 
