DE. JOULE ON SOME THEEMO-DTNAMIC PEOPEETIES OF SOLIDS. 
101 
In order to turn the above into thermal measure, the gutta percha was plunged into 
water of different temperatures, deep enough to immerse the part in which the thermo- 
electric junction was imbedded ; the former plan of operation being inapplicable in this 
instance, on account of the low conducting power of the substance used. It was found 
in this way that the quantities of thermal effect due to the above deflections were 
— 0°-0284 and — 0°-0624, quantities, however, which are likely to err in defect by a very 
small quantity, owing to the nature of the method of finding the value of the deflec- 
tions. The theoretical results, assuming as I did for the metals that the expansion of 
gutta percha by heat is the same whether suffering tension or not, are 
H= 
276-4 —70 
1390 
X 
6354 ^-402 ^•178“ 
and 
H= 
276-4 -150 
1390 
6354 ^'402 ^*178 
-0°*0306, 
:_0°-0656. 
32. The values just given for the expansion and specific heat of gutta percha are those 
which I arrived at by experiments on a part of the same sheet out of which the cylinder 
above used was made. The method pursued in obtaining the expansion was one I have 
frequently found very convenient. It consists in weighing the body in water at two 
temperatm-es, one as much below as the other is above the point of maximum density. 
In this way I found the linear expansion between 2°-4 and 6°-8 to be per 1°, and 
the specific gravity to be 1-00462 at 4°. The specific heat at 4‘’-6 was found by the 
method of mixtures to be -402. 
33. India-rubber . — The extraordinary physical properties of this substance appear to 
have been first remarked by Mr. Gough*. By placing a slip of it in slight contact with 
the edges of the lips, and then suddenly extending it, he experienced a sensation of 
warmth arising from an augmentation of the temperature of the rubber ; and then by 
allowing the slip to contract again, he found that this increase of temperature could be 
destroyed in an instant. In his next experiment, he found that “ if one end of a slip of 
caoutchouc be fastened to a rod of metal or wood, and a weight be fixed to the other 
extremity, m order to keep it in a vertical position, the thong will be found to become 
shorter with heat and longer with coldf.” The third experiment described by Mr. Gough 
has indicated the means of using this substance in the production of textile fabrics ; he 
says, “ If a thong of caoutchouc be stretched, in water warmer than itself, it retains its 
elasticffy unimpaired; on the contrary, if the experiment be made in water colder than 
itself, it loses part of its retractile power, being unable to recover its former figure ; but 
let the thong be placed in hot water, while it remains extended for want of spring, and 
the heat will immediately make it contract briskly.” 
Memoirs of the Literary and Philosophical Society of Manchester, 2nd Series, vol. i. p. 288. 
t Ibid. p. 292. From the context it appears that the weight was used to give tension, as well as to keep 
the slip vertical. ^ 
MDCCCLIX. p 
