502 LECTURE LII. 



tinued stream, and by others in the form of separate particles, moving, with 

 inconceivable velocity, at great distances from each other. 



The circumstances which have been already stated respecting the produc- 

 tion of heat by friction, appear to afford an unanswerable confutation of 

 the whole of this doctrine. If the heat is neither received from the surround- 

 ing bodies, which it cannot be without a depression of their temperature, 

 nor derived from the quantity already accumulated in the bodies themselves, 

 which it could not be, even if their capacities were diminished in any ima- 

 ginable degree, there is no alternative but to allow that heat must be actu- 

 ally generated by friction ; and if it is generated out of nothing, it cannot 

 be matter, nor even an immaterial or semimaterial substance. The colla- 

 teral parts of the theory have also their separate difficulties : thus, if heat 

 were the general principle of repulsion, its augmentation could not diminish 

 the elasticity of solids and of fluids ; if it constituted a continued fluid, it 

 could not radiate freely through the same space in different directions ; 

 and if its repulsive particles followed each other at a distance, they would 

 still approach near enough to each other, in the focus of a burning glass, to 

 have their motions deflected from a rectilinear direction. 



If heat is not a substance it must be a quality ; and this quality can only 

 be motion. It was Newton's opinion, that heat consists in a minute vibra- 

 tory motion of the particles of bodies, and that this motion is communicated 

 through an apparent vacuum, by the undulations of an elastic medium, 

 which is also concerned in the phenomena of light. If the arguments 

 which have been lately advanced, in favour of the undulatory nature of 

 light, be deemed valid, there will be still stronger reasons for admitting this 

 doctrine respecting heat, and it will only be necessary to suppose the vibra- 

 tions and undulations, principally constituting it, to be larger and stronger 

 than those of light, while at the same time the smaller vibrations of light, 

 and even the blackening rays, derived from still more minute vibrations, 

 may, perhaps, when sufficiently condensed, concur in producing the effects 

 of heat. These effects, beginning from the blackening rays, which are in- 

 visible, are a little more perceptible in the violet, which still possess but a 

 faint power of illumination ; the yellow green afford the most light ; the 

 red give less light, but much more heat, while the still larger and less fre- 

 quent vibrations, which have no effect on the sense of sight, may be sup- 

 posed to give rise to the least refrangible rays, and to constitute invisible 

 heat. 



It is easy to imagine that such vibrations may be excited in the component 

 parts of bodies, by percussion, by friction, or by the destruction of the 

 equilibrium of cohesion and repulsion, and by a change of the conditions 

 on which it may be restored, in consequence of combustion, or of any other 

 chemical change. It is remarkable that the particles of fluids, which are 

 incapable of any material change of temperature from mutual friction, 

 have also very little power of communicating heat to each other by their 

 immediate action, so that there may be some analogy, in this respect, 

 between the communication of heat and its mechanical excitation. 



The effects of heat on the cohesive and repulsive powers of bodies have 



