52 THEORY OF HEAT. [CHAP. I. 



constant flow F, in a solid formed of the same substance, the 



F a-b w a-b 

 equation ^ - - or H A . 



J\. & 6 



The value of F denotes the quantity of heat which, during 

 the unit of time, passes across a unit of area of the surface taken 

 on a section parallel to the base. 



Thus the thermometric state of a solid enclosed between two 

 parallel infinite plane sides whose perpendicular distance is e, 

 and which are maintained at fixed temperatures a and b, is 

 represented by the two equations : 



b a a-b ^ T ^dv 



v = a + z t and F=K- - or F=-K-^. 



The first of these equations expresses the law according to 

 which the temperatures decrease from the lower side to the 

 opposite side, the second indicates the quantity of heat which, 

 during a given time, crosses a definite part of a section parallel 

 to the base. 



69. We have taken this coefficient K, which enters into 

 the second equation, to be the measure of the specific conduci 

 bility of each substance ; this number has very different values 

 for different bodies. 



It represents, in general, the quantity of heat which, in a 

 homogeneous solid formed of a given substance and enclosed 

 between two infinite parallel planes, flows, during one minute, 

 across a surface of one square metre taken on a section parallel 

 to the extreme planes, supposing that these two planes are main 

 tained, one at the temperature of boiling water, the other at 

 the temperature of melting ice, and that all the intermediate 

 planes have acquired and retain a permanent temperature. 



We might employ another definition of conducibility, since 

 we could estimate the capacity for heat by referring it to unit 

 of volume, instead of referring it to unit of mass. All these 

 definitions are equally good provided they are clear and pre 

 cise. 



We shall shew presently how to determine by observation the 

 value K of the conducibility or conductibility in different sub 

 stances. 



