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reason for supposing that radiant heat is identical with light, and that 

 they both consist of vibrations of the ultimate particles of matter. 



There is a strong presumption of this from the facts, that every 

 body heated to a certain temperature, dependent only on the nature 

 of the surface, emits light as well as heat ; and that " whenever light 

 manifests itself, heat appears along with it" (Kelland) : the difference 

 between radiant heat and ordinary heat is, that radiant heat is due 

 to vibrations in planes normal to its direction of propagation, and 

 that ordinary heat consists of vibrations in all three dimensions. 



The author endeavoured to show, in the first place, that the mo- 

 tions of the particles of matter, which must be caused by friction, or 

 in the union of two gases in combustion, is sufficient of itself to 

 account for the following phenomena of heat : — 



I. That a body once heated continues of the same temperature, 

 with the exception of heat lost by radiation, conduction, &c. This 

 follows immediately from the principle, that in any system of par- 

 ticles held together by mutual attractions and repulsions, the vis viva 

 is independent of the time, and depends merely on the position of 

 the particles. 



II. That bodies expand b)^ heat. 



Before proceeding to this, the author argued that in gases the 

 repulsive force varies inversely as the cube, and not, as usually stated, 

 the simple power of the distance ; that it is not true, without some 

 limitation, that the force varies as the inverse first power, was urged 

 from the fact that such a force would decrease more slowly than one 

 varying as the inverse square, and consequently would be the force 

 observed in astronomical phenomena ; and even the oxygen of the 

 ocean would repel that of the air instead of attracting it. That the 

 force varies as the inverse cube was deduced from the law of elas- 

 ticity, that the density varies as the pressure ; for if a particle repels 

 other particles with a force varying as the inverse cube, it repels a 

 fixed plane of them with a force varying as the inverse first power. 

 That this is the case may be seen, by considering that though the 

 particle repels particles similarly situated with a force varying as the 

 inverse cube, yet the number of such particles varies directly as the 

 square of the distance, and therefore the whole effect upon the plane 

 varies inversely as the first power. And if this is true for a plane, 

 it is also true for the solid side of the containing vessel ; for any 

 solid may be considered as made up of a succession of planes. 



The law being the inverse cube, it follows that in any position the 

 sum of the forces exerted by any particle on two particles, one on 

 each side of it, is least when that particle is half-way between them, 

 and increases the further the particle is removed from the middle 



point. This is seen directly, for the value of - — ¥—r- -+- 



least when z=0, and increases until x=a. And therefore, in order 

 to produce the same force, it would he necessary that the mean 

 distance should be increased ; and hence if the particles of any aeri- 

 form body be in motion, the force exerted by them would be greater 

 than when at rest ; that is, if the pressure to be supported be con- 



