^'54 tECTURE Lll, 



whole of this doctnne. If the heat is neither received from the suirouiuHng 

 bodies, which it cannot be without a depression of their temperature, nor 

 derived from the quantity aheady accumulated in the bodies themselves, 

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

 able degree, there is no alternative but to allow that heat must be actually 

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

 matter, nor even an immaterial or semimaterial substance. The collateral 

 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 respect- 

 ' ing heat, and it will only be necessary to suppose the vibrations and undula- 

 tions, 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 

 raySjderived 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 invisible, are a little more perceptible 

 in the violet, which still possess but a faint power of illumination; the 

 yellow green afibrd the most light; the red give less light, but much more 

 heat, while the still larger and less frequent vibrations, which have no effect 

 on the sense of sight, may be supposed 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 



