PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 817 



paths, the law of Mariotte, viz.: that at equal temperatures the 

 volume occupied by the same quantity of gas is in inverse propor- 

 tion to the pressure it exerts or sustains, is a direct consequence or 

 corrollary of the hypothesis here advanced. We have only to sub- 

 stitute equal momenta for equal temiJeratures, and it evidently fol- 

 lows that the aggregate pressure must, in a given volume, be pro- 

 portional to the number of atoms, or conversely, the volume occu- 

 pied by a given number of atoms must be inversely proportional 

 to the pressure. 



The development and absorption of heat by the compression 

 and expansion of gases is another evident consequence of this 

 hypothesis. B}^ compression the atoms are brought under a more 

 powerful gravitating influence, which increases the momentum, or 

 in other words, the temperature of the gas. In expansion exactly 

 the reverse of this takes place, the atoms being withdrawn to a 

 greater mean distance from each other, they are less reinforced by 

 gravity, and their momenta or temperatures are reduced. 



The latent heat which is developed in the change from the gase- 

 ous to the liquid state, and absor])ed in the reverse process, is due 

 to a similar cause, as will be more fully seen when we investigate 

 these transformations. 



The remarkable law discovered by Dulong and Petit, that the 

 specific heat of elementary bodies is inversely as their atomic 

 weights, may be considered as direct evidence in support of the 

 hypothesis that heat is momentum. For if we admit this hypothe- 

 sis, remembering that the measure of specific heat is not one of 

 absolute force or momentum, but of vis viva or icorh done in the 

 separation of atoms, or the elevation of their temperature, it obvi- 

 ously follows that more of it will I)e required to produce the same 

 momentum in a light atom Ihan in a heavy one, in the exact ratio 

 of the increase of velocity imparted. This is just what Dulong 

 and Petit have, by careful experiment, established in regard to 

 many substances, both simple and compound. 



To understand this clearly, we must remember that momentum 

 or quantity of motion, which is here made sjaion^-mous with tem- 

 perature, when referred to atoms, is measured hj taking the pro- 

 duct of the weight into the velocity, while vis viva, or work, is 

 proportioned to the space effect produced, or to be produced, as in 

 j-aising Aveights, and is as the weight into the square of the velo- 

 city. So that if we divide the vis viva of a light body or atom by 

 [Am. Inst.] ZZ 



