PhYSICS OF THE NINETEENTH CENT. HEAT. 523 



platinum is placed, it is as quickly raised to a glowing temperature. 

 The stream can also be made to produce such mechanical movements 

 within the tube as would be caused by a stream of particles discharged 

 from the negative pole. 



We pass on now to record some of the most important scientific 

 work of the present century, namely, the establishment of the Dyna- 

 mical Theory of heat, and the determination of the "Mechanical 

 Equivalent" Dr. Young, in commenting upon Davy's experiment of 

 rubbing two pieces of ice together (page 310), pointed out in 1807 

 that the heat in this case cannot be received from surrounding bodies, 

 as no fall of their temperature occurs, nor can it be derived by any 

 conceivable diminution of " capacity for heat " from heat accumulated 

 in the ice. It must therefore be admitted that the heat is actually 

 generated by the friction. If it be thus generated it cannot be ma- 

 terial, for this would amount to the creation of matter out of nothing. 

 As heat cannot therefore be a substance, it must be a quality, and this 

 quality can only be motion. It was indeed Newton's opinion (and had 

 been that of others) that heat consists in a minute but rapid vibratory 

 movement of the particles of bodies. This motion may be communi- 

 cated through an apparent vacuum by undulations of the same elastic 

 medium which is concerned in the phenomena of light. " If the 

 arguments which have been lately advanced in favour of the undula- 

 tory nature of light be deemed valid, there will be still stronger reasons 

 for admitting this doctrine respecting heat, and it will only be neces- 

 sary to suppose the vibrations 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, where 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 possesses but a faint power 

 of illumination ; the yellow-green afford the most ; 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." Young considered that according to this view of heat its 

 phenomena had in several respects much analogy to those of sound. 



In spite of the clear and forcible manner in which Young showed 

 the inadequacy of the doctrine that regarded caloric as material or 

 quasi-material, that theory prevailed, and was generally entertained by 

 chemists and physicists until nearly the middle of the present century. 

 The reason of this may be found in the facility and convenience with 

 which it lent itself to the expression of the phenomena of latent and 

 specific heat. Before the new doctrine of heat was systematically 

 formulated, however, there were not wanting speculations on the con- 

 nection and mutual convertibility of heat and mechanical action. 



