Preliminary Remarks. $ 



stantly observed in bodies. We accordingly infer that there is 

 a general cause of interior repulsion in bodies, by which the 

 attractive forces are continually balanced. This cause which 

 resides in all bodies, seems to be referrible to the principle of 

 heat. The particles of each body, actuated at the same time 

 by these two opposite forces, naturally put themselves in a state 

 of equilibrium, resulting from a compensation of energies, and 

 they approach and recede, according as the forces to which they 

 are exposed from without favor the attractive or repulsive 

 principle. It is with these minute bodies as it is with the plan- 

 ets of our system, which are found to move and oscillate, as it 

 were, in orbits of variable forms and dimensions, without the 

 system being destroyed, or the general equilibrium being dis- 

 turbed. From these different conditions of equilibrium arise, as 

 we shall see hereafter, all the secondary and changeable forms 

 of bodies, such, for example, as are denominated aeriform, liquid, 

 and solid, crystallized, hard, elastic, &c. 



8. In all the phenomena which present themselves, the 

 particles of matter act, or rather are acted upon, as if they were 

 perfectly inert, that is, deprived of all power of self-direction. 

 They can be moved, displaced, stopped, by causes foreign to 

 themselves, but we never have been able to discover the least 

 trace of any thing like choice or will proper to the particles 

 themselves. If the ball which rolls upon a billiard table in con- 

 sequence of the impulse that is given to it, loses by little and 

 little its velocity, and at length comes to a state of rest, it is 

 entirely the effect of the continual resistance that it meets with 

 from the roughness of the cloth with which it comes in contact, 

 and from the particles of the air through which it passes. Make 

 the cloth more smooth, or the air more rare, and the same impulse 

 would keep it longer in motion ; substitute for the cloth a marble 

 slab highly polished, or a band of stretched wire, the elasticity 

 of which is still more perfect, and the ball would continue its 

 motion for a much longer time ; from all which it is to be infer- 

 red, that if the obstacles were completely removed, there would 

 be no diminution of the velocity first communicated, and the 

 motion would never cease. A stone thrown from the top of a 

 tower, and urged at the same time by the impulse of the hand 

 and by gravity, will come to the ground after proceeding a 

 certain distance, losing at the same time its horizontal velocity, 

 by imparting it to the particles of air against which it impinges-. 



