PHYSICAL FORCES 93 



were, immersed in another sort of fluid substance of an 

 indefinitely more refined nature, called ether a substance 

 tending to separate the particles (and therefore to oppose 

 gravity), and also possessed of and able to transmit 

 various orders of vibratory motions. Motion so un- 

 imaginably minute taking place amongst the particles, 

 or "molecules," of bodies is termed molecular motion. 

 It is thus distinguished from the motion of bodies we 

 can perceive, which is called molar motion, or the motion 

 of perceptible masses of matter. 



Heat is now treated as if it consisted of such molecular 

 oscillations, which are conceived of as varying in extent 

 and velocity, but as continuing perpetually and tending 

 to become everywhere uniform by intercommunication 

 between all particles of all bodies without limit. Such 

 intercommunication is supposed to take place through 

 the hypothetical refined substance, called ether, which is 

 conceived of as being something essentially different from 

 the solids, liquids, and aeriform bodies, of which we have 

 hitherto spoken, and also conceived of, as being universally 

 diffused. For some scientific purposes, it is most conveni- 

 ent to suppose this ether to be an ideally perfect fluid ; 

 while for others it is treated as an ideal jelly-like sub- 

 stance.* Men of science are of course quite free to treat 

 it in any fashion which may help on investigation, but we 

 must not regard such speculative hypotheses as repre- 

 senting real, ascertained truths. Moreover, as before said, 

 an elementary work like this is not the place to treat of 

 such a problem as the question what "heat" in itself may 

 be. We must be content with serviceable hypotheses. 

 One such is that it consists of molecular vibrations varying 

 in intensity and capable of propagation through space in 



* See ante, p. 90. 



