Mr. A. A. Harrison on the Theory of Heat, 401 



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 th^ 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 phaenomena of radiation, it may be shown 

 from theoretical considerations that the inverse cube is the law 

 required. Tlie 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 sether 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 

 has been shown that the ultimate particles of our atmosphere com- 

 pose such an aether. But if our atmosphere is the luminiferous aether, 

 we must next inquire whether it does pervade space. Omitting 

 variations of temperature, and merely considering the atmosphere as 

 subject to the two forces of elasticity and gravity, we have for the 

 equation of a column of air on a unit of surface, 



dp=-—<— pdz, or — P-z=z-i. — , where A;=i-, 

 ^ z'' ^ pdz z""- p 



Integrating this, we find that^ and p, though they become extremely 

 small, never vanish ; and therefore, if these laws are absolutely true, 

 our atmosphere does pervade space. 



It may be well to obviate the objection, that black substances 

 radiate heat best, and white substances light. This arises from 

 employing the same word radiation to denote two different things : 

 by radiated heat is meant heat given out from a heated body ; by 

 radiated light is meant the secondary radiation from the surface of 

 a body exposed to light. 



If Sir J. Leslie's experimental calculation of the heat lost from 

 the sun be correct, there is no need of any theories to account for its 

 generation. 



