Essay on Motion and Force. 259 



motion, or motion of agglomerated molecules, retaining 

 during their motion their agglomerated condition. The 

 only non-molecular changes which matter in the latter 

 condition is known to undergo, are change of position 

 and change of form. In change of position all the parts 

 of a body move in the same direction ; in change of form 

 some, or all of the parts of a body, which are then to be 

 regarded as separate bodies, change their position ; therefore 

 as change of position is the definition of motion, we have all 

 change which masses undergo as masses, reduced to mo- 

 tion. 



In considering molecular motions, I will select heat as a 

 type of all motions of the class. With regard to heat the 

 opinions of the mass of physicists have within a short time 

 undergone a complete revolution. From having been con- 

 sidered an imponderable form of matter, it came to be 

 considered as force, and lastly as a mode of molecular mo- 

 tion. I think I do not make a mistake in assuming that 

 the last mentioned opinion is the one most generally en- 

 tertained by physicists at the present time. I will quote 

 from Grove. 



"Let us now divest the mind of the impression that heat 

 is in itself any thing substantive, and suppose that these 

 phenomena are regarded for the first time, and without any 

 preconceived notion on the subject; let us introduce no 

 hypothesis, but merely express the facts of which we have 

 become cognizant ; to what do they amount ? to this, that 

 matter has pertaining to it a molecular repulsive power, a 

 power of dilatation, which is communicable by contiguity or 

 proximity. 



Heat, thus viewed, is motion, and this molecular motion 

 we may readily change into motion of masses, or motion in 

 its most ordinary and palpable form ; tor example, in the 

 steam engine, the piston and all its concomitant masses of 

 matter are moved by the molecular dilatation of the vapor 

 of water." The same view is adopted by Tyndall, who has 

 made it the subject of a course of lectures, in which accu- 



