Heat across Layers of Gas. 429 



6. Some idea will be formed of the quantity of beat wbich 

 will pass from A to B by tbe process here describedj and for 

 which I would suggest the name penetration, by forming an 

 expression which aims at roughly representing the quantity of 





heat absorbed by the gas per second from a square centimetre 

 of A. One such expression is approximately* 



clQ ^j AOi ,.. 



_=^crp,^, (^) 



in which V is the velocity with which the adjustment is made, 

 a the heat which would raise a gramme of the gas one degree 

 in temperature, and pi the density (referred to water) of the 

 gas where it is in contact with A. 



To get the loss by penetration per second from the whole 

 surface of the cooling body, we have to find the value of the 



integral j - 



dA, clA being an element of the surface of the 



cooling body, and -p having the value assigned to it above. 



If the surface is everywhere equally exposed (a condition easily 

 secured in making experiments with thermometer-bulbs), this 



* To make the formula accurate, we should write \ -w taken between 

 the limits ^^ and 6^ - a9^, instead of -^, which is approximately its value 

 when A^^ is small compared with 6^. 



