106 



'I'lial the sl(';ul\' slalc is I'cacln'd ; .'I. 'I'lial the malcrial of 

 the wall is lioin()i;('iu'()iis ; 4. Tlial llic wall is iiol lliiiilcd In 

 any (lirrctiou oliu'r than liu' diri'dioii of How of heal. In 

 any application of tlit' i'ormulas these assumptions need lo 

 be considered. 



It should be noticed that the specific heat of the mate- 

 rial does not i)lay any role in the flow' of heat, in the steady 

 state. This is of importance in several ])ro])lems, as wiien, 

 for example, the passage of heat tlii'on,i;h water is to be 

 compared with the passage of heat through ice, two snl)- 

 stances which possess a very different specific heat. 



In any practical case, the wall is in contact w'itli two 

 media, one on the warmer side, the other on the cooler 

 side. TIeat Hows from the warmer medium to the cooler 

 one through the wall. But the constant temperature of 

 these media (even if they are perfectly stirred) is not the 

 temperature at the surfaces of the wall. In other w^ords, 

 when two bodies are in contact, their surfaces of contact 

 are not at the same temperature or, what is equivalent, 

 the interface between two substances in contact has a heat 

 conductivity (sometimes called * 'contact-conductivity") 

 which is different from that of either of the two substances. 

 For having overlooked this point in his experiments on the 

 heat conductivity coefficient in metals, Peclet, a pioneer 

 physicist of the last century, was led to the evidently 

 erroneous conclusion that all the metals have the same con- 

 ductivity. Applications of the notion of contact-conductiv- 

 ity in biological investigations will be mentioned later. 



Usually it is not with a single wall in contact with two 

 media that one has to deal but with several walls in contact 

 with one anotlier. Furthermore, these walls are rarely 

 limited by plane surfaces, they have any shape. Of the 

 numerous problems which result from the coml)ination of 

 walls of various forms, the simpler ones, in particular that 

 of the "multiple cylindrical wall" and that of tlic "nni]ti])l(' 

 spherical wall," ai'e studied in the treatises on mathe- 

 matical physics. 



As an application of the problem of the multiple 



