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THERMOTICS PROPER. 



CHAPTER I. 

 THE DOCTRINES OF CONDUCTION AND RADIATION. 



Section. 1. Introduction of the Doctrine of Con- 

 duction. 



BY conduction is meant the propagation of heat 

 from one part to 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 communi- 

 cated 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 



