84 VISCOSITY OF GASES 215 



warmer by 1 degree. According to Stewart and T ait's 

 observations, the emissivity of a surface of bare aluminium 

 is four times smaller, so that, in accordance with Kirch- 

 hoff's law, it seems just to assume that a vane made of 

 aluminium will absorb in the same time only a fourth part 

 of that amount of heat, i.e. about 0*00004 calorie. The 

 difference between the two amounts of heat received, viz. 

 0*00011 calorie, which is the energy that drives the radio- 

 meter, is capable of raising O'OOOll x 425 = 0*05 gram by 

 1 metre, or 5 grams by 1 cm. [that is, the power is 5 gm. cm. 

 per sec.]. This energy, acting on a vane, would certainly 

 be capable of turning the light-mill round, and, indeed, even 

 if the difference in temperature of the vanes and envelope 

 were 100 or 1,000 times smaller than 1 degree. For with the 

 mill there moves only the rarefied, and therefore light, air 

 which has to overcome at the wall only an external friction, 

 which is very much diminished by the rarefaction. 



85. Influence of the Temperature on the Viscosity 



Since according to the kinetic theory the friction of gases 

 is to be looked upon as a consequence of the molecular 

 motion, the coefficient of friction must be variable with the 

 temperature if that theory is true ; and we may expect, from 

 the reasons given in 76, that it increases with rise of 

 temperature. Every observer who has investigated the 

 influence of the temperature on the friction has found 

 this expectation justified. Only in respect of the rate of the 

 increase with the temperature have the results of different 

 observers shown differences which were at first hard to 

 explain. 



We can only conclude, 1 from the observations of 

 Graham 2 on the flow of gases through tubes, that the 

 friction of gases really increases with the temperature, as the 

 theory requires ; we might draw the further conclusion from 

 them that the increase of friction with the temperature is in 

 nearly the same ratio for all gases. Both speak in favour 

 of the theory ; for the speed of the molecules, which comes 



1 Pogg. Ann. 1866, cxxvii. p. 369. 2 Phil Trans. 1846 and 1849. 



