But to return to molecules. Molecules are endowed with an inherent faculty of motion; only under the conditions of what we call the solid, they are so compressed, that there is no room for any motion appreciable to the senses. Even if the solid is converted into vapour, the molecules are still much restrained in their movements by the pressure of the air. But of late years, great improvements (partly chemical, partly mechanical) have been made in producing perfect _vacua_; that is to say, in getting glass or other vessels to be so far empty of air, that the almost inconceivably small residue in the receptacle has no perceptible effect on the action of a small quantity of any substance already reduced to the form of gas or vapour introduced into it. Dr. W. Crookes has made many beautiful experiments on the behaviour of the molecules of attenuated matter in _vacua_. The small quantity of vapour introduced contains only a relatively small number of molecules, which thus freed from all sensible restraint within the limits of the glass vessel used, are free to move as they will; they are observed to rush about, to strike against the sides of the vessel, and under proper conditions to shine and become _radiant_, and to exhibit extraordinary phenomena when subjected to currents of electricity. So peculiar is the molecular action thus set up, that scientific men have been tempted to speak of a fourth condition of matter (besides the three ordinary ones, solid, liquid, and gaseous), which they call the ultra-gaseous or radiant state of matter. This marvel of molecular structure seems already to have removed us sufficiently far from the idea of a simple inert mass, which might be primordial and self-caused. But we have not yet done. Even imagining the extreme subdivision[1] of the particles in one of Dr. Crookes' vacuum globes, the particles are still water. But we know that water is a compound substance. The molecule has nine parts, of which eight are hydrogen and one oxygen--because that is the experimentally known proportion in which oxygen and hydrogen combine to form water. As we can (in the present state of our knowledge) divide no farther, we call these ultimate fragments of simple or elementary substance _atoms_. [Footnote 1: As to the possibility of _indefinite_ subdivision of matter, see Sir W. Thomsons's lecture, _Nature_, June, 1883, _et seq._]