Atomic Constitution of Elastic Fluids. 35 



more reason to anticipate the existence of such a relation be- 

 tween the number of molecules and the absolute or specific 

 heat, than between that number and the heat of temperature. 

 Upon this principle, since unequal increments of absolute heat 

 are required to effect equal expansions, we should conclude 

 that the numbers of atoms are also unequal. 



That equal increments of temperatures should affect all 

 elastic fluids in the same degree, is a manifest consequence of 

 the constitution of such fluids. The unequal expansibility of 

 bodies in the solid and liquid states is to be ascribed to the 

 interference of the attractive forces which maintain those con- 

 ditions of matter, and which counteract, with energies vary- 

 ing in different bodies and at different distances, the repulsive 

 agency of heat. But the molecules of elastic fluids are se- 

 parated to such a distance from one another that their mutual 

 attractions become insensible *. They are therefore sub- 

 jected to the undisturbed influence of repulsive forces. Ac- 

 cording to the theory of Laplace, caloric constitutes the sole 

 agent of repulsion; and equal increments of temperature, being 

 identical with equal increments of elasticity, are necessarily 

 followed by equal expansions. " Sous une pression constante la 

 densite d'un gaz etant, comme on Pa vu, reciproque a cette 

 fonction de la temperature, son volume est proportionnel a cette 

 fonction...la temperature est alors representee par ce volume, 

 et ses variations sont representees par les variations du volume 

 d'un gaz soumis a une pression constante." 



It cannot be requisite to pursue this argument further, since 

 it has been shown by Laplace, that both the law of Mariotte, 

 and that of equal expansion discovered by Dalton and Gay- 

 Lussac, are mathematically derivable from the following sup- 

 positions: — that the molecules of gases are at such a distance 

 that their mutual attractions are insensible; — that these mole- 

 cules retain caloric by a principle of attraction ; — that their 

 mutual repulsion is due to the repulsion of the molecules of 

 caloric; — and, finally, that this repulsion is only sensible at 

 imperceptible distances. If these suppositions be conceded, 

 the laws of Mariotte and of Dalton are susceptible of rigid 

 demonstration, and are moreover applicable to all elastic fluids, 

 whatever be the nature or the number of their molecules. 



There is a third argument noticed by Dr. Prout, to which 

 M. Dumas attaches much weight, in support of the doctrine 

 of equality of atoms in a given volume. It is founded on the 

 recent experiments of MM. Delarive and Marcet, which show 

 that all the gases, in equal volumes, have the same capacity 



* Mccanique Celeste, livre xii. ch. i. torn. v. p. 89-91. 

 F2 



