94 ELEMENTS OF SCIENCE 



all direction?. Some facts with regard to heat accord 

 best with that conception which regards it as a peculiar 

 kind of fluid. Other facts fit in best with the hypothesis 

 that it consists of molecular motions ; while yet other 

 facts seem to require a union of both these hypotheses. 

 Such matters, however, can be little more than glanced 

 at here, though it is necessary to bring some representa- 

 tion of molecular motion and of ether, before the mind 

 of any one who desires to become acquainted with the 

 elements of science. This is the more indispensable 

 because there are definite quantitative relations between 

 molar motion and the molecular motion of heat. The 

 heat produced in iron by the strokes of the smith's 

 hammer, and the occasionally setting on fire of wheels 

 produced by their revolution (when friction much 

 impedes motion) roughly show this ; but careful investi- 

 gations have now revealed precise numerical equivalents 

 between heat and the motions of bodies. 



Leaving all hypotheses on one side, we will now con- 

 sider the six various phenomena of heat we enumerated 

 before we began to speak of heat in itself. 



(i) Conduction. When two bodies of different tem- 

 peratures are placed in contact, the hotter body becomes 

 cooler, and the cooler body becomes hotter, till both are 

 of the same temperature. When they are bodies equal 

 in mass and of the same substance, the increase of tem- 

 perature in the one will be equal to the decrease of 

 temperature in the other. Heat is thus conducted from 

 one body to another. This transference of heat 

 takes time, but the time varies greatly according to the 

 nature of the substance; some substances being much 

 better conductors of heat than others are. Thus even a. 

 short piece of charcoal burning at one end can be held 

 in the fingers at its other end without inconvenience ; 



