64 
which brings us to Mr. Spencer’s fourth and last great argu- 
ment against creation, derived from the persistence of force. 
28. This principle he describes as “the ultimate of ulti- 
mates” (p. 169). It cannot be proved experimentally, because 
this could only be done by weighing or measuring, in which 
processes it must be assumed, before any result can be relied 
upon, that both the force of gravity and the quantity of the 
matter which constitutes the weight, remain unaltered. 
Neither can it be proved a priori , because it is the most 
general of all principles, and while it comprehends all other 
principles, is itself contained in none (First PrincipJes f 
pp. 192 b and 192 c, 8rd ed.). Since, then, it cannot be 
proved either experimentally or a priori , it must, he argues, 
be an axiom. Now, what I have already said about the 
alleged axiom that matter is indestructible applies equally to 
this. I cannot myself see it to be an axiom, because I per- 
suade myself that I can very well conceive its contradictory 
to be true. I can conceive terrestrial gravity to diminish, 
just as I can conceive the caloric resident in a heated body to 
diminish by radiation. It is generally believed that solar heat 
is gradually diminishing from that cause. Why, then, should 
it be inconceivable that solar or terrestrial attraction might in 
some similar way diminish ? Let it be remembered that the 
question before us is not whether this be a fact or no, but 
whether it is thinkable — whether it can be mentally pictured ; 
for if it can, its contradictory is not an axiom. As to the fact. 
Professor Challis has shown that gravitation can be accounted 
for on the hypothesis of a reaction of the atoms of which 
matter is composed against aethereal pressure. If that be the 
actual cause of it, it appears to me that the attraction of any 
particular mass, such as the sun or the earth, would not 
diminish or increase, because, according to his theory, the 
atoms always continue to be of the same size and shape (being 
absolutely incompressible), and there seems to be no reason 
why the pressure of the aether upon them, and consequently 
their reaction against it, should alter. But whether this be 
so or not, the contrary is as conceivable as it is that heat 
should radiate. It should be observed that the theory of 
Professor Challis, although it is, if true, an important advance 
in hydrodynamical science, does not in the least vitiate what 
has been said by Mr. Herbert Spencer as to the inconceiva- 
bility of the manner in which gravitation acts, owing to there 
being always intervals between the atoms of the aether. 
Professor Challis distinctly says of this aether, that it is 
“itself atomically constituted” (Transactions of the Victoria 
Institute , vol. XII., p. 7) ; and more fully he says in the 
