36 CORRELATION OF PHYSICAL FORCES. 



mechanical work, it would pro tanto be consumed, and could 

 not restore the temperature to the dilated air ; but if it per- 

 form no work, no heat is lost. Mr. Joule has experimentally 

 proved this proposition. 



In commencing the subject of heat, I asked my reader to 

 put out of consideration the sensations which heat produces 

 in our own bodies : I did this because these sensations are 

 likely to deceive, and have deceived many as to the nature 

 of heat. These sensations are themselves occasioned by 

 similar expansions to those which we have been considering ; 

 the liquids of the body are expanded, i.e. rendered less viscid 

 by heat, and from their more ready flow we obtain the sen- 

 sation of agreeable warmth. By a greater degree of heat, 

 their expansion becomes too great, giving rise to a sense of 

 pain, and if pushed to extremity, as with the heat which pro- 

 duces a burn, the liquids of the body are dissipated in vapour, 

 and an injury or destruction of the organic structure takes 

 place. A similar though converse effect may be produced by 

 intense cold ; the application of frozen mercury to the animal 

 body produces a burn similar to that produced by great heat, 

 and accompanied with a similar sensation. 



Doubtless other actions than those above-mentioned inter- 

 fere in producing the sensations of heat and cold ; but I 

 think it will be seen that these will not affect the arguments 

 as to the nature of heat. The phenomenal effects will be 

 found unaltered : heat will still be found to be expansion, cold 

 to be contraction ; and the expansion and contraction are, as 

 with the two bladders of air, correlative i.e. we cannot ex- 

 pand one body, A, without contracting some other body, B ; 

 we cannot contract A without expanding B, assuming that we 

 view the bodies with relation to heat alone, and suppose no 

 other force to be manifested. 



I have said that there are a few exceptions as to heat being 

 always manifested by an expansion of matter. One class of 

 these exceptions is only apparent : moist clay, animal or vege- 

 table fibre, and other substances of a mixed nature, which 

 contain matter of diverse character, some of which is more 

 and some less volatile, i.e. expansible, are contracted on the 

 application of heat ; this arises from the more volatile matter 



