QQ^ PROPEETV OP INDIAN RUBBER. 



and lefo denfe. I made a piece of Caoutchouc a little heavier than an equal 

 bulk of water, the temperature of which was 45 degrees: 

 the veflel containing the refin and water was then placed on 

 the (ire; and when the contents of it were healed to 130 

 degrees, the Caoutchouc floated on the furface. 

 It becomes cold Exp. 1. Hold one end of the flip, thus prepared, between 

 i^g^outlnd hTt ^^'^ ^^""^^ ^"'* fore-finger of each hand ; bring the middle of 

 .1^ contiaftion. the piece into flight contafl with the edges of the lips ; * taking 

 care to keep it ftraight at the time, but not to ftretch it much 

 beyond its natural length: aftfer taking thefe preparatory fteps, 

 extend the flip fuddenly ; and you will immediately perceive 

 a fenfation of warmth in that part of the mouth which touches 

 it, arifing from an augmentation of temperature in the Caout- 

 chouc : for this refin evidently grows warmer the further it is 

 extended ; and the edges of the lips poflefs a high degree of 

 fenfibility, which enables them todifcover thefe changes with 

 greater facility than other parts of the body. The increafe of 

 temperature, which is perceived upon extending a piece of 

 Caoutchouc, may be defl:royed in an inflant, by permitting 

 the flip to contraQ again ; which it will do quickly by virtue 

 of its own fpring, as oft as the ftretching forth ceafes to a6t as 

 foon as it has been fully exerted. Perhaps it will be faid, 

 that the preceding experiment is conduced in a negligent 

 manner; that a perfon, who wiflies for accuracy, vvill not 

 truft his own fenle of feeling in inquiries of this defcription, 

 but will contrive to employ a thermometer in the bufinefs. 

 Should the objedion be flarted, the anfwer to it is obvious ; for 

 the experiment in its prefent ftate demonftrates the reality of 

 3 fingular fa6l ; by convincing that fenfe, which is the only 

 dire^ judge in the cafe, that the temperature of a piece of 

 Caoutchouc may be changed, by compelling it to change its 

 dimenfions. The ufe of a thermometer determines the rela- 

 tive magnitudes of thefe variations, by referring the quefiioa 

 of temperature to the eye ; experiments of this fort are there- 

 fore of a mathematical nature, and afFord a kind of know- 

 ledge with which we have, nothmg to do at prefent ; for we 

 are not inquiring after proportions, but endeavouring to efta- 



* This efFe£l was firft noticed in 17S4', at Mr. Kirwap's meetings 

 ' in Newman ftreet, and Dr. Crawford afcribed it to change of 



capacity fimilar to what he fiippofed to take place in a nail by 

 hammering.— N. ' 



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