106 THEORY OF HEAT. [CHAP. II. 



of which is below the plane and the other above it, are unequally 

 heated; the first, whose temperature is highest, must therefore 

 send to the second, during each instant, a certain quantity of heat 

 which, in some cases, may be very small, and even insensible, 

 according to the nature of the body and the distance of the two 

 molecules. 



The same is true for any two other points whatever separated 

 by the plane. That which, is most heated sends to the other 

 a certain quantity of heat, and the sum of these partial actions, 

 or of all the quantities of heat sent across the plane, composes 

 a continual flow whose value does not change, since all the 

 molecules preserve their temperatures. It is easy to prove that 

 this floiv, or the quantity of heat which crosses the plane M during 

 the unit of time, is equivalent to that luhich crosses, during the same 

 time, another plane N parallel to the first. In fact, the part of 

 the mass which is enclosed between the two surfaces M and 

 N will receive continually, across the plane M, as much heat 

 as it loses across the plane N. If the quantity of heat, which 

 in passing the plane M enters the part of the mass which is 

 considered, were not equal to that which escapes by the opposite 

 surface N, the solid enclosed between the two surfaces would 

 acquire fresh heat, or would lose a part of that which it has, 

 and its temperatures would not be constant; which is contrary to 

 the preceding lemma. 



135. The measure of the specific conducibility of a given 

 substance is taken to be the quantity of heat which, in an infinite 

 solid, formed of this substance, and enclosed between two parallel 

 planes, flows during unit of time across unit of surface, taken 

 on any intermediate plane whatever, parallel to the external 

 planes, the distance between which is equal to unit of length, 

 one of them being maintained at temperature 1, and the other 

 at temperature 0. This constant flow of the heat which crosses 

 the whole extent of the prism is denoted by the coefficient K, 

 and is the measure of the conducibility. 



136. LEMMA. If we suppose all the temperatures of the solid in 

 question under the preceding article, to be multiplied by any number 

 whatever g, so that the equation of temperatures is v = g gz, 

 instead of bsing v = 1 z, and if the two external planes are main- 



