DE. JOULE ON SOME THEEMO-DTNAMIC PEOPEETIES OP SOLIDS. 
93 
obvious that a gas might be conceived as so constituted that the heat evolved by its 
compression would be in no respect the equivalent of the mechanical force employed, 
and that therefore we had no right to assume such equivalency except as a hypothesis 
to be tested by experiment. Accordingly after this hypothesis had been proved by me 
approximately*, Professor Thomson devised the experiments f by which we have 
succeeded in defining the limits of its accuracy. The same philosopher has also applied 
his powerful analysis to the investigation of the thermo-elastic properties of matter If;. 
The results of the experiments I have just given an account of can only be considered as 
negative, but it has been decided by Professor Thomson that the compression of a spring 
gives a certain, though excessively small thermal effect, owing to the almost exact coun- 
terpoise of heating and cooling effects on the compressed and extended sides, The 
method of obtaining appreciable results was obviously to examine these opposite effects 
separately. I have, therefore, on the suggestion of Professor Thomson, undertaken 
some experiments with a 'view to ascertain the heat developed by longitudinal compres- 
sion, and that absorbed on the application of tensile force. 
6. At the outset it was obvious that a very delicate test of temperature would be 
required, and no means appeared to offer so many advantages as that of thermo-electri- 
city. Professor J. D. Fokbes had constructed a thermo-multiplier capable of detecting 
temperatures not exceeding one thousandth of a degree Fahrenheit. Adopting some 
of the refinements introduced by Melloni and Forbes, I have simplified the instrument 
so as to render its construction and management very easy, and also increased its sensi- 
bility by immersing it into the vacuum of an air-pump. My thermo-multiplier is repre- 
sented by the adjoining sketch (fig. 2), where a is the air-pump firmly clamped to a 
strong stool, the legs of which pass through holes in the 
laboratory floor and are driven into the ground beneath; 
S is a glass chimney receiver ; c a block of wood supported 
on feet which rest on the pump plate ; d a piece of glass 
rod fixed to the block, over which is thrown the filament 
which supports the astatic needles. Two thick copper 
wires (e) dip into mercury cups formed in the block, and, 
being carried out of the receiver through holes drilled in 
the ground glass plate f, are bent into two mercury cups 
placed on the top of the instrument. Pitch was employed 
to close all orifices air-tight. 
7. The details of the astatic needles, which are poised 
according to the plan first suggested by Professor Thomson, 
will be better understood by inspecting fig. 3. The needles 
Proceedings of the Eoyal Society, June 20, 1844; and Philosophical Magazine, May 1845. 
t Philosophical Magazine, 1852, Supplement; Philosophical Transactions,- 1853, Part III. p. 357, and 
1854, Part II. p. 321. 
X Quarterly Mathematical Journal, April 1855. 
mdccclix. 
Pig. 2, 
0 
