The tensions of the aggregated molecules of space are thus counterbalanced only so long as all space is equally occupied and a state of perfect quiescence exists in its every part. A molecular disturbance in one part is immediately communicated to adjacent parts, and finally to all. With the first movement, gravity asserts itself, for gravity exists and must exist in all parts, and must actively manifest itself whenever the perfect mutual balance of space is disturbed and a center of energy developed, and co-ordinately with the action of gravity begins that of electricity. Movements among the molecules are converted into movement of mass; centripetal motion begets condensation, this begets sensible heat and vortical movement; then come the phenomena of electrical generation by moving contact with the gases of space, then repulsion and disassociation of the elements of the aqueous vapors, combination of simple into compound elements; and, the balance once disturbed, the state of unstable equilibrium is forever destroyed, and all space henceforth must exhibit constant change. There are whole segments of space absolutely blank, so far as visible systems are concerned, which seem to have been exhausted, for the present æons at least, to supply material for the vast adjacent galaxies which extend along their borders; see illustrations in Proctor's "Essays on Astronomy," article "Distribution of the Nebulæ." It need not be supposed that such stage of perfect and universal quiescence ever existed in fact; it is like the Nirvana of the Buddhist philosophers,--a subjective and not an objective condition. We can have no knowledge of the existence, even, of material things, save from their phenomena, the manifestation of interchanging forces, upon which rests our threefold basis of knowledge, perception, cognition, and comparison. We know nothing of matter, except as affected by internal or external force, nor of force itself, except as it acts in one mode or another upon matter. All beyond this is, for us, without form and void.