HEAT. 49 



power of reaction of the compressed against the dilated air. 

 If the heat due to compression be made to perform mechani 

 cal work, it would pro tanto be consumed, and could not 

 restore the temperature to the dilated air ; but if it perform 

 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 simi 

 lar 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 sensation 

 of agreeable warmth. By a greater degree of heat, their ex 

 pansion becomes too great, giving rise to a sense of pain, and, 

 if pushed to extremity, as with the heat which produces 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 four d 

 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 expand 

 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 few exceptions as to heat being 

 3 



