THEKMOTICS PROPER, 



CHAPTER I. 

 THE DOCTRINES OF CONDUCTION AND RADIATION. 



Section 1. Introduction of the Doctrine of Conduction. 



BY conduction is meant the propagation of heat from one part '.o 

 another of a continuous body ; or from one body to another in 

 contact with it ; as when one end of a poker stuck in the fire heats 

 the other end, or when this end heats the hand which takes hold of 

 it. By radiation is meant the diffusion of heat from the surface of a 

 body to points not in contact. It is 'clear in both these cases, that, in 

 proportion as the hot portion is hotter, it produces a greater effect in 

 warming the cooler portion ; that is, it communicates more Heat to it, 

 if Heat be the abstract conception of which this effect is the measure. 

 The simplest rule which can be proposed is, that the heat thus com- 

 municated in a given instant is proportional to the excess of the heat 

 of the hot body over that of the contiguous bodies ; there are no 

 obvious phenomena which contradict the supposition that this is the 

 true law ; and it was thence assumed by Newton as the true law for 

 radiation and by other writers for conduction. This assumption was 

 confirmed approximately, and afterwards corrected, for the case of 

 Radiation ; in its application to Conduction, it has been made the 

 basis of calculation up to the present time. We may observe that 

 this statement takes for granted that we have attained to a measure 

 of heat (or of temperature, as heat thus measured is termed), corre- 

 sponding to the law thus assumed ; and, in fact, as we shall have 

 occasion to explain in speaking of the measures of sensible qualities, 



