4'28 Prof. Norton on Molecular Physics. 



substances. There is nothing in the conception we have formed 

 of a molecule, and of the probable difference of physical condi- 

 tion in molecules of different substances, that is apparently op- 

 posed to this result. 



The capacity for heat of compound is in general greater than 

 that of simple molecules, and is greater in proportion as the 

 molecule is more complex. This indicates that when the tem- 

 perature is raised 1°, a certain portion of the heat is expended 

 in urging asunder the constituents of the molecules, and that this 

 portion is greater in proportion as the molecule is more complex. 



Heat-rays of different rates of vibration, — The calorific spec- 

 trum shows that the heat emitted from a hot body is composed 

 of rays of an infinite variety of rates of vibration between certain 

 limits. The physical cause of this fact will appear if we reflect 

 that the heat-rays have their origin in the vibrations of the 

 atomettes of the molecular atmospheres, and that these are situ- 

 ated at every variety of distance from the central atoms, between 

 certain limits. For the circumstances of equilibrium of these 

 atomettes are different, and their rates of vibration when dis- 

 placed should be different. The fact that the most intense heat- 

 rays, in ordinary cases of combustion, are those of low refrangibi- 

 lity and the phenomena of the evolution of calorific and luminous 

 rays when a body is heated to incandescence, indicate that the 

 electric atomettes which are nearest the central atoms have the 

 lowest rate of vibration. 



We have seen that the expansive action of heat is a necessary 

 consequence of the fundamental principle that the heat-pulses 

 constitute a repulsive force, and that they are absorbed, more or 

 less, by the molecular atmospheres which they encounter. It 

 may be urged as an objection to the notion that radiant heat is 

 a repulsive force, that bodies when heated do not exert any 

 sensible repulsive action upon other contiguous bodies. We are 

 not prepared to admit that experiment has furnished no evidence 

 of such repulsive action under any circumstances ; but the entire 

 absence of such action upon bodies of sensible magnitude would 

 in fact be no decisive proof that waves of radiant heat do not 

 convey a series of preponderating repulsive impulses; for if 

 these impulses penetrate to the atoms of the molecules, they 

 should be ultimately taken up by their atmospheres, and ex- 

 pended as an expansive force upon these atmospheres, and in 

 urging the molecules asunder; and if they do not reach the 

 atoms, no motion should be imparted to them. Since it is im- 

 probable that the more intense impulses of heat will be wholly 

 absorbed by the particles immediately at the surface of the body 

 upon which they fall, a direct repulsive action of the heat may 

 take effect to a certain depth below the surface. Have we not 



