82 FRAGMENTS OF SCIENCE. 



thus apart and then liberate them. They clash together, 

 but, by virtue of their elasticity, they quickly recoil from 

 each other, and a sharp vibratory rattle succeeds their col- 

 lision. This experiment •will enable you to figure to your 

 mind a pair of clashing atoms. We have, in the first place, 

 a motion of the one atom toward the other — a motion of 

 translation, as it is usually called. But when the atoms 

 come sufficiently near each other, elastic repulsion sets in, 

 the motion of translation is stopped and converted into a 

 motion of vibration. To this vibratory motion we give the 

 name of heat. Thus, three things are to be kept before 

 the mind — first, the atoms themselves ; secondly, the force 

 with which they attract each other ; and thirdly, the mo- 

 tion consequent upon the exertion of that force. This mo- 

 tion must be figured first as a motion of translation, and 

 then as a motion of vibration ; and it is not until the mo- 

 tion reaches the vibratory stage that we give it the name 

 of heat. It is this motion imparted to the nerves that pro- 

 duces the sensation of heat. 



It would be useless to attempt a more detailed descrip- 

 tion of this molecular motion. After the atoms have 

 been thrown into this state of agitation, very complicated 

 motions must ensue from their incessant collision. There 

 mast be a wild whirling about among the molecules. For 

 some time after the act of combination this action is so 

 violent as to prevent the molecules from coming together. 

 The water is maintained for a time in a state of vapor. 

 But as the vapor cools, or in other words loses its mo- 

 tion, the water molecules coalesce to form a liquid. And 

 now we are approaching a new and wonderful display of 

 force. No one who had only seen water in its vaporous or 

 liquid form could imagine the existence of the forces now 

 to be referred to ; for as long as the substance remains in 

 a liquid or vaporous condition, the play of these forces is 

 altogether masked and hidden. But let the heat be gradu- 



