H MATTER, LIVING FORCE AND HEAT. 



to cause them to attract one another through a greater 

 space. We may conceive, then, that the communication of 

 heat to a body consists, in fact, in the communication of 

 impetus, or living force, to its particles. It will perhaps 

 appear to some of you something strange that a body 

 apparently quiescent should in reality be the seat of 

 motions of great rapidity ; but you will observe that the 

 bodies themselves, considered as wholes, are not supposed 

 to be in motion. The constituent particles, or atoms of the 

 bodies, are supposed to be in motion, without producing a 

 gross motion of the whole mass. These particles, or atoms, 

 being far too small to be seen even by the help of the most 

 powerful microscopes, it is no wonder that we cannot 

 observe their motion. There is therefore reason to suppose 

 that the particles of all bodies, their constituent atoms, are 

 in a state of motion almost too rapid for us to conceive, for the 

 phenomena cannot be otherwise explained. The velocity 

 of the atoms of water, for instance, is at least equal to a 

 mile per second of time. If, as there is reason to think, some 

 particles are at rest while others are in motion, the velocity 

 of the latter will be proportionally greater. An increase of 

 the velocity of revolution of the particles will constitute an 

 increase of temperature, which may be distributed among 

 the neighbouring bodies by what is called conduction — that 

 is, on the present hypothesis, by the communication of the 

 increased motion from the particles of one body to those of 

 another. The velocity of the particles being further in- 

 creased, they will tend to fly from each other in consequence 

 of the centrifugal force overcoming the attraction subsisting 

 between them. This removal of the particles from each 

 other will constitute a new condition of the body — it will 

 enter into the state of fusion, or become melted. But, from 



