512 THE POPULAR SCIENCE MONTHLY. 



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 de- 

 sire to break away from conventional lines, but it is in strict ac- 

 cord 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 re- 

 gard the heat of a body as the aggregate kinetic energy of the mole- 

 cules. 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 objectiv- 

 ity 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 con- 

 stitute electric atmospheres and magnetic effluvia, to convey sen- 

 sations 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 exer- 

 cise that we can appreciate the horror of ethers which sober- 



