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MR. J. P. JOULE ON HEAT, 



The experiments on the changes of temperature pro- 

 duced by the rarefaction and condensation of air, give Uke- 

 wise an insight into the constitution of elastic fluids ; for 

 they show that the heat of elastic fluids is the mechanical 

 force possessed by them ; and since it is known that the 

 temperature of a gas determines its elastic force, it follows 

 that the elastic force, or pressure, must be the effect of the 

 motion of the constituent particles in any gas. This motion 

 may exist in several ways, and still account for the pheno- 

 mena presented by elastic fluids. Davy, to whom belongs 

 the signal merit of having made the first experiment abso- 

 lutely demonstrative of the immateriality of heat, enunciated 

 the beautiful hypothesis of a rotary motion. He says, "It 

 seems possible to account for all the phenomena of heat, if 

 it be supposed that in solids the particles are in a constant 

 state of vibratory motion, the particles of the hottest bodies 

 moving with the greatest velocity and through the greatest 

 space : that in fluids and elastic fluids, besides the vibra- 

 tory motion, which must be considered greatest in the last, 

 the particles have a motion round their own axes with dif- 

 ferent velocities, the particles of elastic fluids moving with 

 the greatest quickness ; and that in ethereal substances the 

 particles move round their own axes, and separate from 

 each other, penetrating in right lines through space. Tem- 

 perature may be conceived to depend upon the velocity of 

 the vibrations ; increase of capacity on the motion being 

 performed in greater space ; and the diminution of tem- 

 perature during the conversion of solids into fluids or 

 gases, may be explained on the idea of the loss of vibratory 

 motion, in consequence of the revolution of particles round 



communicated by Professor Thomson to the Royal Society of Edinburgh, 

 In this paper the very important law is established, that the fraction of 

 heat coDverted into power in any perfect engine, is equal to the range of 



temperature divided by the highest temperature above absolute zero 



May, 1861. J. P. J. 



