171 



stant, the average distance of the particles must increase, and the 

 body must expand. 



III. That every aeriform body not in contact with a liquid expands 

 in the same proportion. This was accounted for by the circumstance, 

 that the increase of pressure depends only upon the ratio of the dis- 

 turbance to the original distance, and not at all upon the absolute 

 distance. 



IV. That air and elastic fluids give out heat on compression. By 

 conrpression the absolute distance of the particles from one another 

 is diminished ; but the absolute motion remaining the same, the rela- 

 tive motion is increased. 



V. That the same amount of heat is generated in two gases sub- 

 jected to the same pressure ; for the absolute distance of the particle 

 in both being diminished in the same proportion, and the absolute 

 motion remaining unaltered, the relative motion is increased in the 

 same proportion in both. 



VI. The specific heats are inversely as the atomic weights. Here 

 it was necessary to show that mass is not necessarily proportional to 

 the quantity of matter, as usually stated ; or rather, that a body may 

 have a different mass when considered with regard to the molecular 

 force from what it has with respect to the force of gravity. With 

 regard to elasticity of gases, the weight of any single particle is so 

 small as not to affect the result. The question remains, whether 

 we know anything of the masses of different particles relatively to 

 this repulsive force. To determine their masses we have these data. 

 In several different gases equivalent volumes under the same pressure 

 occupy the same space, that is (assuming the Daltonian theory, that 

 equivalent volumes contain the same number of particles), that each 

 particle of the two different gases exerts an equal pressure on the 

 adjacent particles : and hence, with reference to this law, the mass 

 of a single particle in each of these two different gases is the same, 

 and therefore the " vis viva " of equivalent weight or volumes subject 

 to the same motion is the same for both ; that is, the quantity of 

 heat of an equivalent of each is the same, and therefore the specific 

 heat of a given weight is inversely as its equivalent number or atomic 

 weight. 



With reference to the phenomena of radiation, it may be shown 

 from theoretical considerations that the inverse cube is the law 

 required. The inverse first is impossible, for then there could be no 

 vibrations. For the same reason the inverse second is impossible 

 (Camb. Phil. Trans, vol. vii. p. 98). The inverse fourth is also im- 

 possible, for then there could be no vibrations, and the velocity 

 would be infinite (vol. vi. p. 325). It has also been shown that 

 neither the second nor the fourth would satisfy the conditions of the 

 equations (vol. vii. p. 419). Hence, from the theory of radiation, 

 it is supposed that the luminiferous eether consists of solid particles, 

 attracting one another with a force varying as the inverse square 

 (vol. vii. p. 110), and repelling with a force varying as the inverse 

 cube. 



Now from the Daltonian theory, and the law of elastic fluids, it 



