HEAT. 39 



Thus not only heat or the expansive force given to other 

 bodies by a body cooling would be given out by water 

 freezing, but also the force due to the expansion in the body 

 itself, and force would thus seem to be got out of nothing ; 

 but if water in a confined space be gradually cooled, the ex- 

 pansion attendant on its cooling as it approaches the freezing 

 point would occasion pressure amongst its particles, and thence 

 tend to antagonise the force of dilatation produced in them 

 by cooling, or to resist their tendency to freeze ; or in other 

 words the pressure would tend to liquefaction, and conversely 

 to the usual effect of pressure, produce cold instead of heat, 

 and thus neutralise some of the heat yielded by the cooling 

 body. Hence we find that it requires a lower temperature to 

 freeze water under pressure than when exempt from it, or 

 that the freezing point is lowered as the pressure increases for 

 bodies which expand in freezing an effect first predicted by 

 Mr. J. Thomson, and experimentally verified by Sir W. 

 Thomson ; while, as shown by M. Bunsen, the converse effect 

 takes place with bodies which contract in freezing ; here the 

 pressure co-operates with the effects of cold, both tending to 

 approximate the particles, and such substances solidify at a 

 higher temperature in proportion as the pressure is greater ; 

 so that we might expect a body of this class, which under the 

 ordinary pressure of the air is at a temperature just above its 

 freezing point, to solidify by being submitted to pressure 

 alone, the temperature being kept constant. 



A similar class of exception to the general effect of heat 

 in expanding bodies is presented by vulcanised caoutchouc. 

 This has been observed by Mr. Gough, and, indeed, wag 

 pointed out to me many years ago by Mr. Brockedon, to be 

 heated when stretched, and cooled when unstretched. 



Mr. Joule finds that its specific gravity is less when 

 stretched than when unstretched, and that when heated in its 

 stretched state it shortens, presenting in this particular condi- 

 tion a similar series of relations to those which are presented 

 by water near or at its freezing point. 



With the exception of this class of phenomena, which offer 

 difficulties to any theory which has been proposed, the general 

 phenomena of heat may, I believe, be explained upon a purely 



