pended as subtitles to more general terms expressing forms of energy, and appended in a way that would permit them to be dropped altogether without detriment to the treatment of their phenomena. Not that the phenomena are different from what they were in former times, but they have become much more effectually correlated in a general scheme of energy. Such a mode of presenting the subject might be ascribed to a mere desire to break away from conventional lines, but it is in strict accord with the work and conclusions of physicists generally in the last quarter of a century and especially within the last decade. Physicists accept fully the mechanical theory of heat. They regard the heat of a body as the aggregate kinetic energy of the molecules. They accept in general the kinetic theory of gases, but are not uniform in their views as to the extension of this theory to liquids and solids. In the mechanical theory of heat, however, the idea that all the molecules of all bodies are in motion is fundamental. Nowadays, instead of ascribing phenomena to the action of mysterious "forces," with perhaps a force of one kind for gravity, of another kind for thermal or electric or magnetic effects, and treating force as a real agent bringing about changes, it is the custom to recognize in any body or system of bodies a certain quantum of energy of which the form or distribution is altered by a change in the form or configuration of the body or system of bodies. Energy is the thing studied and force is merely the rate at which the energy of a body is altered in comparison with the change in the position or shape of the body. The term force is still in use for convenience and brevity, but the objectivity of force has disappeared. Force is not a real thing at all, but energy, like matter, has an objective existence. Also, when force was regarded as an agent, it was discussed as acting at a distance without regard to a medium for transmitting action from one body to another, or, as we now say, for conveying energy. But if bodies possess and exchange energy, and energy is only perceived by us in connection with matter, we find it proper not only to recognize a medium throughout space, but to discuss the forms in which energy exists in that medium, which is spoken of as ether. The idea of such a medium is not modern. After pointing out that the hypothesis of an ether was a device often resorted to for the purpose of mystification as much as explanation, Maxwell says: "Ethers were invented for the planets to swim in, to constitute electric atmospheres and magnetic effluvia, to convey sensations from one part of our bodies to another, and so on, until all space had been filled three or four times over with ethers. It is only when we remember the extensive and mischievous influence on science which hypotheses about ethers used formerly to exercise that we can appreciate the horror of ethers which sober-
