490 Mr. W. Sutherland on Thermal 



p ... -. 1000 526 330 314 205 147 95 



real 2 10 11 21 31 48 



def. force \ , _ _ D -. ■ 00 ., 



lexp 1 3 5 8 14 28 44 



p 59 41 26-5 20 12 8 5 



real 63 70 64 58 42 30 20 



• ° rCe lexp 64 70 66 58 45 37 29 



In view of the experimental uncertainty already pointed out 

 at the high pressures and that which is to be proved at the 

 lowest pressures, this comparison shows that the modified 

 formula represents the facts for hydrogen about as well as 

 possible. 



We will now compare the theoretical values of the para- 

 meters c' and A" with the numerical ones just obtained ; c' 

 stands for &K(r 2 — i , i)/(i , 2 + l 'i)> m which K is proportional to D 

 and is a function of k' ; also (v2~" v i)/( v 2 + v i) * s the same 

 for all the gases, so that c' is proportional to D. But 

 A /r =dD q v 9 /\6if(v2^-v i y, and v varies as m~*, and therefore 

 (A ,/ 7)' 2 /m}i is proportional to D, which now means distance 

 from edge of vane to which transpiration extends. The 

 following table contains 10 7 &', c f , and (AS'if/m)* for com- 

 parison ; 7] in terms of that for 2 as 1, and m in terms of that 

 for H 2 as 2 are appended : — 



H 2 . Air. 2 . N 2 . CO. C0 2 . 



Wk' 3324 480 563 524 510 317 



c' 416 30 30 20 132 125 



(AV/m»)5 -762 -520 -468 -520 '411 -284 



v -44 -90 10 -87 -87 -755 



m 2 28-8 32 28 28 44 



It can be seen that on the whole these numbers confirm the 

 theoretical conclusion that thermal transpiration near the 

 edge of the plate is effective to a distance which increases 

 with the conductivity. The smallness of c f for N 2 as com- 

 pared to its value for air and the smallness of c' for CO are 

 points that require confirmation by experiments with an 

 apparatus lending itself better to quantitative calculations than 

 the torsion radiometer of Crookes. 



The values of W" , as they represent only a small amount of 

 slipping, and are not given very definitely by the experiments, 

 are not worth further consideration. 



So much for what may be called the static form of the 

 radiometer ; of the results obtained from a great variety of 

 moving radiometers constructed by Crookes the following are 

 the most important. In a radiometer containing two flies, 

 one pivoted above the other, and having their blackened 



