28 
to the earth. We know that some highly elastic matter must exist and fill 
the whole space between us and the sun, in order to convey the light and 
heat so essential to the development of life on the earth. But beyond this 
we know nothing of it.” To these remarks I reply as follows : It is the 
very principle of my argument to begin with regarding the ether and its 
qualities as “ altogether hypothetical.” But remembering Newton’s rule of 
not making gratuitous suppositions “ contrary to the tenour of experience ” 
and “ the analogy of nature,” I take account from the first, in making the 
hypotheses, of the same ground of necessity as that adduced by Mr. Brooke 
for assuming the existence of some highly elastic matter by means of which 
light and heat are conveyed to us from the sun. So far, therefore, I can 
perceive no difference between Mr. Brooke’s views and mine, excepting that 
I give a specific name to the elastic substance, and call it ether. I admit, 
however, that from this point I proceed to make particular hypotheses 
respecting the ether, as, that it presses, and that its pressure is always pro- 
portional to its density. Now these hypotheses are justifiable, as hypotheses, 
on the ground that they give the means of testing the reality of the ether 
and its assumed properties, by being appropriate foundations of mathe- 
matical reasoning for deducing results that may be compared with experi- 
mental facts. There are departments of physical science in which advance 
can be made only by proceeding according to this method of hypotheses. 
And although by such a method the hypotheses are not absolutely proved to 
be realities, a moral certainty that they are such is established in proportion 
to the number and the variety of the explanations they give of phenomena. 
Since, in my opinion, the mathematico-physical science of the present day 
has established a moral certainty of the reality of the ether, and of its being 
such as for the purpose of theoretical research it is assumed to be, I am 
unable to admit that, because we cannot apply experimental tests to it as we 
do to other material substances by seeing and handling them, we know 
nothing about it. It is true that we can never “ see ” it, because, being the 
means by which grosser bodies are seen, it is itself invisible. I think Mr. 
Brooke is hardly consistent in saying that we never “ feel ” it, because he 
admits (second page of Discussion) that sound and light are subjective sen- 
sations ; and since we may be said to feel the air in our sensation of sound, 
we may with as good reason be said to feel the ether in the sensation of light. 
We have not the power, neither have we any need, to “test” its presence 
by seeing or handling it, inasmuch as the lightning flash, and the distant 
star, attest its presence near us, as well as in the remotest regions of space. 
We cannot “weigh” it, because, being the cause of all weight, it is itself 
imponderable. 
With reference to the assertion in sec. 25, that “ it must be by means of 
the ether that force is exerted when of our own will, under conditions and 
limitations of organization, we move our limbs, or set in motion any ex- 
traneous body,” Mr. Brooke remarks that the act of volition has an ante- 
cedent cause to the exertion of force.” I agree so far with this view iis to 
admit that volition, by whatever cause determined, is antecedent to the 
