IS KAJUnXD OASES ABISING FROM INEQUALITIES OF TEMPERATURE. 



To return to the case of transpiration through a capillary tube. When 

 the temperature is uniform 



By experiment* on capillary tubes of glass, MM. Eundt and Warburg 

 found* for the value of G for air at different pressures and at from 17 C. 

 to 2TC, 



Q 



G = - centimetres (79) 



P 



where p is the pressure in dynes per square centimetre, which is nearly the 

 une as in millionths of an atmosphere. For hydrogen on glass 



15 

 G = centimetres (80). 



P 



When there is no flow of gas in a tube in which the temperature varies 

 from end to end, the pressure is greater at the hot end than at the cold 

 end. Putting Q = we have 



d& =6 p0 



M* 



The quantity 6 ~. is just double of that calculated in section (3) of the 



introduction, and is therefore in C.G.S. measure 0'63 xp for dry air at 15 C. 

 Let us suppose a = 0'01 centimetre, and the pressure 40 millimetres of mercury, 

 then </' ='00016 centimetre. 



If ono end of the tube is kept at O'C. and the other at 100C., the 

 pressure at the hot end will exceed that at the cold end by about 1-2 

 millionths of an atmosphere. 



The difference of pressure might be increased by using a tube of smaller 

 bore and air of smaller density, but the effect is so small that though the 

 theoretical proof of its existence seems satisfactory, an experimental verification 

 of it would be difficult. 



* Pogg. Ann., July, 1876. 



