104 DE. JOtTLB ON SOME THEEMO-DYNAMIC PEOPERTIES OP SOLIDS. 
40 Although my experiments, already detaUed, on the thermal effects of sttetching 
JtrlLr were not, on account of the imperfect elasticity of that substance at low tern- 
peratures, well calculated to give exact numerical results, the 
from them is perfectly in accordance with Professor Thomson s formula where, m thi. 
Stance a, the expansion, being negative, the result indicated is a rise of temperature on 
the application of the tensile force. . . r cA-mo fimp it has 
41 When, by keeping india-rubber at rest at a low temperature or 
become rigid, it ceases to be heated when stretched by a weight, and, on the ““““I; a 
Lding elct takes place as in the metals and gutta percha. The expenment by w^ch 
I ascertained this fact was made with a piece of india-rubber t o ™ ™ r this 
had been exposed for several days to a temperature not ° f a,fl!.rion 
state it was found that the application of a tensile force of 1 s pio uce ® 
of 20’, indicating cold, and that a contrary deflection, mdicating heat, ™ P™™ . • 
removing the weight. After raising the temperature of the rubber to lb , the e as cut 
of the rubber was restored, and the reverse phenomenon of heat on stretchmg pioduce , 
indicated by a deflection of 15'. . i t- o rv-f thh 
42. “rmlcmized" Indm-mbier.—l observed the principal physical chaiacteis of 
substance before I was acquainted with Mr. Gouoh’s discovery of the properties of simp e 
india-rubber, above noticed. The superior permanency of the elasticity of lubber 
combination with sulphur, and its unimpaired existence at low temperatuies, len ei« 
it better qualified for experiments in which accurate numerical results were esue . 
43 I determined the specific heat of vulcanized india-rubber from portions ol tne 
specimens used in the experiments about to he detailed. The method of mnxtures was 
employed, a quantity in smaU bits being raised to a given temperatiue in a bath ol an, 
and then plunged into a thin copper can partly filled with cold water. The result, lelia 
to at least one- tenth of its amount, was 0’415. ^ ^ ^ , 90.05 
44. The expansion hy heat was found by weighing in "vsater ^ ^ ^ non “or 
below the maximum density. The result showed a cubical dilatation of O'OOOo.fi per 
degree Centigrade, an expansion greater than that of any other so i ii ei o examii • 
45 Mr. Godsh has stated that the specific gravity of mdia-rubber is increased . 
stretching it ; and to this he attributes the heat thereby evolved. The expenment, how- 
ever is a very delicate one, and it does not appear that this philosopher possessed he 
mea’ns of arriving at a reliable result. My experience wnth vulcanized ” 
leads me to doubt the accuracy of Mr. Gough’s conclusion. I weighed elastic b. s 
alternately stretched and unstretclied in water. The result of the first series oi experi- 
ments wal, that the bands unstretched had a specific granty -99600 1 but when puller 
twice their natural length, -994641. The second series gave me for the specific giavity 
of the unstretched bands -990918, but when pulled to two and a half times the natui. 
length -988483. The same vulcanized india-rubber bands were used in both series, ami 
the diminution of the specific grarity in the second series is owing to the constant loss 
of sulphur taking place. On account, however, of the altemation ot the expeiinien s 
