It may be regarded as an act of presumption in an obscure critic to offer an opinion of the theories of these great masters in science. We venture, however, to suggest that most men will find a difficulty in conceiving how space absolutely full of matter can be made to contain more, or how a truly continuous substance can be capable of condensation. The most tenuous ether, if it be absolutely continuous, occupies the whole of the space in which it lies--that is, there is no point of the space which is not occupied by a point of matter.[285] But the hardest iron can do no more than this, and, therefore, on this hypothesis it seems impossible to account for its greater density. It is suggested that if molecules are mere assemblages of parts of the ether moving in a peculiar way, then greater density may be due to a modification in the motion of molecules, and not merely to the greater frequency of the eddying molecules in a given space. But how can a truly continuous substance have _parts_, and how can relative motion occur in an absolute plenum? The very notion of particles is quite inconsistent with the continuity of matter; and in a universe absolutely full no motion whatever would be possible. We are told that Sir William Thomson and Professor Tait find no difficulty in all these, to our minds, contradictory conceptions, and therefore we must conclude that our intellect is not properly "focussed so as to give definition without prenumbral haze." Granting, then, the absolute continuity of all matter, and the possibility of motion in an absolute plenum, the question which concerns us most in this essay is, How is motion generated and sustained? One of the greatest lights of this new school tells us that "all we can affirm of matter is that it is the _recipient_ of impulse and of energy."[286] They no longer regard the atom "as a mystic point endowed with inertia and the attribute of _attracting_ and _repelling_ other such centres with forces depending on the intervening distances."[287] They have "no faith whatever in _attractions_ and _repulsions_ acting at a distance between centres of force."[288] Force, then, is not regarded by these leading physicists as an inherent attribute of matter. The primary fluid, originally inert and motionless, must have been set in motion by some force, by some agency external to and distinct from itself. An "original impetus" _from without_, according to Maxwell,[289] or a "pressure" of the universal ether "from somewhere _outside_ the world of stars," according to Challis,[290] must be the source of all motion and all forms of energy in the universe.