PHENOMENA OF HEAT AND SOUND. 71 



Aristotelians, and, in their discussions on heat, they attached 

 great importance to it. According to Boyle, they expressed 

 Aristotle's opinion in the short Latin formula, " congregare 

 liomogenea et segregare heterogenea." * There are many 

 exceptions to the truth of this general formula, but such 

 exceptions were not understood by the Aristotelians. Heat 

 does sometimes bring together substances of the same kind 

 and separate those of different kinds, e. g., when a mixture 

 of pieces of copper and lead is melted together with a flux 

 in a crucible, for the molten product will form three well- 

 defined layers, the lowest containing nearly all the lead and 

 the middle one nearly all the copper. When, however, 

 sulphur is dropped on a bar of white-hot iron, resulting 

 in a union of these unlike substances, and when water is 

 vaporized by heating it, the Aristotelian formula does not 

 hold good. 



The acuteness of the sensation of heat or cold produced 

 when the hand is placed in contact with a body depends 

 largely upon the conductivity of the body and its heat 

 capacity. Copper or mercury, for instance, produces a more 

 acute sensation than wood at the same temperature. Aris- 

 totle's ideas on this subject are very imperfect, and not 

 consistent, for he sometimes explains it by relying on differ- 

 ences in certain physical characters of the bodies, and some- 

 times by means of their assumed inherent cold or heat. In 

 some cases, he says, the same substances produce a very 

 cold sensation if deprived of heat, and a burning sensation 

 when heated, the sensation being most acute in the case of 

 bodies which are very hard or solid, e. g., the sensation pro- 

 duced by a hot stone is more acute than that produced by 

 hot water, and that produced by hot water is more acute 

 than that produced by hot smoke or vapour, and similarly 

 when these substances are cold.t In an earlier passage, 

 he assumes that the coldness of bodies is inherent, and 

 makes the coldness of watery and earthy substances depend 

 on his views on the composition of these bodies from 

 his four elements, for both water and earth are defined 

 by the elementary force cold.t Water and substances 

 for the most part of the nature of water, i. e., liquid, were 

 considered by Aristotle to be cold, water being particularly 

 of a nature opposed to that of fire, but substances more of 



'•■ The WorJcs of the Honourable Robert Boyle, new edition, London, 

 1772, vol. i. p. 488. 



f Meteorol. iv. c. 11, s. 8. t Ihid.iv. c. 11, s. 3. 



