These differing degrees of cohesion depend upon temperature, for if the densest and hardest substances are sufficiently heated they will become gaseous. This is only another way of saying that the states of matter depend upon the amount of molecular energy present. Solid ice becomes water by the application of heat. More heat reduces it to steam; still more decomposes the steam molecules into oxygen and hydrogen molecules; and lastly, still more heat will decompose these molecules into their atomic state, complete dissociation. On cooling, the process of reduction will be reversed until ice has been formed again. Cohesive strength in solids is increased by reduction of temperature, and metallic rods become stronger the colder they are. No distinction is now made between cohesion and chemical affinity, and yet at low temperatures chemical action will not take place, which phenomenon shows there is a distinction between molecular cohesion and molecular structure. In molecular structure, as determined by chemical activity, the molecules and atoms are arranged in definite ways which depend upon the rate of vibrations of the components. The atoms are set in definite positions to constitute a given molecule. But atoms or molecules may cohere for other reasons, gravitative or magnetic, and relative positions would be immaterial. In the absence of temperature, a solid body would be solider and stronger than ever, while a gaseous mass would probably fall by gravity to the floor of the containing vessel like so much dust. The molecular structure might not be changed, for there would be no agency to act upon it in a disturbing way. THE ETHER HAS NO CORRESPONDING STATES. Degrees of density have already been excluded, and the homogeneity and continuity of the ether would also exclude the possibility of different states at all comparable with such as belong to matter. As for cohesion, it is doubtful if the term ought to be applied to such a substance. The word itself seems to imply possible separateness, and if the ether be a single indivisible substance, its cohesion must be infinite and is therefore not a matter of degree. The ether has sometimes been considered as an elastic solid, but such solidity is comparable with nothing we call solid in matter, and the word has to be defined in a special sense in order that its use may be tolerated at all. In addition to this, some of the phenomena exhibited by it, such as diffraction and double refraction, are quite incompatible with the theory that the ether is an elastic solid. The reasons why it cannot be considered as a liquid or gas have been considered previously.