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bodies can receive, according to the Professor’s notion, motion 
from this ether, and communicate motion to it. Ether, there- 
fore, he affirms to be a material substance, less dense than that 
with which we are usually familiar, and capable of assuming the 
modes of motion called heat, light, electricity, and magnetism. 
All this sounds as dogmatic and assured as though it were a 
well-ascertained fact, instead of being an effort of the scientific 
imagination, to add a necessary supplement to a favourite 
theory. 
28. We find Mr. Grove decidedly dissenting from it, because he 
believes it an inadequate explanation of the phenomena it was 
invented to explain. He thinks light, for instance, “ results 
from a vibration or motion of the molecules of matter itself, 
rather than from a specific ether pervading it.” And as 
regards heat, he says, — “That the phenomena presented 
by heat, viewed according to the dynamic theory, cannot 
be explained by the motion of an imponderable ether ” (p. 167). 
Again, he writes (p. 168), “An objection that immediately 
occurs to the mind in reference to the ethereal hypothesis of 
light is, that the most porous bodies are opaque ; cork, charcoal, 
pumice-stone, all very porous and very light, are all opaque.” 
The natural objection to Mr. Grove’s theory is, that if these 
forces be the result of molecular action, the space between the 
sun and earth must be a plenum, filled with matter. This he 
supposes it to be, the matter consisting of the atmospheres of 
the planets, very much attenuated, but sufficiently dense to 
transmit these molecular movements. But even this he 
acknowledges to be an assumption, in more modest and 
philosophic words than those used by Professor Tyndall. He 
says, — “ At the utmost, our assumption, on the one hand, is, 
that wherever light, heat, &c., exist, ordinary matter exists, 
though it may be so attenuated that we cannot recognize it by 
the tests of other forces, such as gravitation ; and that to 
expansibility of matter no limit can be assigned. On the 
other hand, a specific matter without weight must be assumed, 
of the existence of which there is no evidence , but in the 
phenomena, for the explanation of which its existence is 
supposed. To account for the phenomena, the ether is 
assumed ; and, to prove the existence of the ether, the 
phenomena are cited. For these reasons, and others above 
given, I think that the assumption of the universality of 
ordinary matter is the least gratuitous.” Each is, therefore, 
an assumption, and a gratuitous one, but that of the ether the 
most so ; and on this most gratuitous assumption the notion of 
the continuity of motion and the persistence of energy is based. 
29. But Mr. Grove is not by any means alone in his objec- 
