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V. On some Thermo-dynamic Properties of Solids. By J. P. Joule, LL.J).., F.R.S., 
F.C.S., Hon. Mem. Phil. Soc. Cambridge, Vice-President of the Lit. and Phil. Soc. 
Manchester, Corresp. Mem. B.A. Sc. Turin, &c. 
Eeceived April 22, — Eead June 10, 1858. 
1. Aetee finding the numerical relation between heat and work in 1843, it immediately 
occurred to me to investigate various phenomena in which heat is evolved by mechanical 
means, and of these one of the most interesting and important appeared to be the evolu- 
tion of heat by the compression of elastic fluids. If the heat given out in this case 
proved to be the equivalent of the work spent, then the natural inference was that the 
elastic force of a gas and its temperature are owing to the motion of its constituent 
particles, both being proportional to the square of the velocity of the particles ; and that 
the force of a falling body employed in compressing a gas is exhibited anew in the form 
of temperature. On the other hand it was possible, secondly, to conceive of an elastic 
fluid which would not give out any heat by compression. This would be the case if it 
was made up of mutually repelling particles, the temperature of which was that of the 
mass. The work required to compress the fluid would be the same on either hypothesis, 
but in one supposition the effect would be developed in actual energy, in the other in 
the potential form. Thfidly, we may suppose a fluid exhibiting as heat a portion of 
the force employed in its compression, and retaining the rest in the potential form. Or 
we may have a fluid giving out more heat than the equivalent of the work spent upon 
it when it is compressed and maintained at a constant Kg. l. 
temperature. Experiment proved that the heat actually 
evolved was very approximately that due to the compress- 
ing force. Nevertheless, it seemed desirable to demon- 
strate the possibility of a gas so constituted that heat shall 
not be evolved by its compression. It occurred to me 
that a bag full of elastic metallic springs would illustrate 
such a gas. If the springs were properly formed, the 
elasticity of the bag would follow the laws of gaseous 
pressure. To the question. Would such a bag evolve 
heat on compression \ I could readily answer, no ; for 
in the bending of a spring one part is extended while the 
other is compressed, and thus it might be expected that 
the thermal effect on the whole would be neutral. Still it seemed desirable to decide 
the point by experiment. 
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