DE. JOIJLE ON SOME THEEMO-DTNAMIC PEOPEETIES OE SOLIDS. 
99 
deflection in the opposite direction, indicating heat, when the weight was removed. 
With 388 lbs., I had 35'-8 and 42'-2 : with 583 lbs., 56'-4 and 57'-8: and with 1166 lbs., 
1° 55'-8 and 1° 58'-6. Hence it appeared that the quantity of cold produced by the 
application of tension was sensibly equal to the heat evolved by its removal ; and further, 
that the thermal effects were proportional to the weights employed. 
21. The above trials having been made without an accurate determination of the 
thermometric value of the deflections immediately afterwards (a practice I found it im- 
portant to follow in order to obviate the effects of any alteration in the magnetic inten- 
sities of the earth, astatic needles, or controlling magnet, which might be taking place), 
I repeated the experiment, using the first method above described, and found a mean 
deflection of 1° 22'-8 with a tension of 775 lbs. The thermometric value of this deflec- 
tion, determined in the manner already explained, was — 0°T15 Centigrade. 
22. Professor Thomson, in his researches on the thermo-elastic properties of metals, 
has demonstrated the following formula as applicable to the phenomenon in question, — 
t p e \ 1 
H=jXfXYX-X-. 
where H is the thermal increase in degree Centigrade, t the temperature Centigrade 
from absolute zero, J the mechanical equivalent of the thermal unit, p the pressure 
applied in pounds, which in the case of tension is of course a negative quantity, e the 
longitudinal expansion per degree Centigrade, 5 the specific heat, and w the weight in 
pounds of afoot length of the bar*. Applied to the above experiment the formula 
gives 
276-7 -775 
^“1390 ^ 1 ^ 
It _ It _ 
81200 ^- 11 ^ 
1568 
= -0°-11017. 
23. In another experiment by the first method, but with another thermo-electric 
junction, I obtained a thermal effect of 0°-124 ; in this case the formula gave 
276 -775 
^“ 1390 ^ 1 ^ 
81200^-11 ^-1568 
= -0°-1099. 
2d . A third experiment, in which the second method was used, and in which I had 
the advantage of Professor Thomson’s assistance, gave a thermal effect of — 0°-1007. In 
this case the formula gave 
287 -725 
^■”1390^ 1 ^ 
81200^-11^-1568 
■ 0°-1069. 
25. Hard Steel . — With stretching weights of 194, 388, and 775 lbs., I observed 
deflections of the needle amounting to 27', 48', and 1° 37'. Another experiment by the 
* The formula signifies in fact that the heat evolved by compressing a solid is equivalent to the work 
required to compress the volume of it due to temperature, just as in the case of a perfect elastic fluid, applied 
to which the expression becomes simplified to Mi. CiiAPEVEOX has given a theoretical estimate of the 
heat disengaged by the cubical compression of iron (Scientific Memoirs, vol. iii. p. 373). 
t Lavoisieu and Laplace. % DtrLOxe and Petit. 
