92 ELEMENTS OF SCIENCE 



- 38. Ice (as before said) melts at 32, but tin needs a 

 temperature of 442 to melt, and antimony requires 

 812. 



We have just seen that heat can disappear and become 

 potential, as also that it can again reappear or be evolved. 

 It can also be (i) conducted, (2) conveyed, (3) radiated, 

 (4) absorbed, (5) reflected, and (6) refracted. The real" 

 nature of heat in itself is absolutely unknown, that is to 

 say it is only known through certain effects itself 

 remaining permanently inaccessible to our senses. 



Various suppositions, or hypotheses, have been 

 suggested as to its nature, the value of which depends 

 on the extent to which any of them can enable us to 

 harmonise, foresee, and predict different actions of 

 various bodies, which are directly perceptible by us. 



It was at one time supposed to be a peculiar kind of 

 substance termed " caloric" But such substance, it was 

 evident, could have no weight, since nothing is made 

 heavier by being heated, or having, as it was supposed, 

 " caloric " added to it. It could also exist without mani- 

 festing itself in any way in the condition just above 

 spoken of* as " latent" or " potential " heat. 



But another hypothesis found favour subsequently; 

 an hypothesis which, it will be seen, is still more useful 

 in reference to the next physical energy we shall consider 

 namely, light. We have saidt that in mechanics it is 

 convenient to suppose each body to consist of a great 

 quantity of minute particles, and the different supposed 

 motions of these supposed particles are taken to explain 

 the diverse conditions we describe as liquid J and aeri- 

 form^ Now we may suppose these particles to be, as it 



* See ante, p. 91. t See ante, p. 43. 



J See ante, p. 68. See ante, p. 78. 



