Transpiration and Radiometer Motion. 489 



p 297 190 171 110 70 48 



real. 13 20 22 30 40 44 



def ' f0rCe - Up. 14 20 21 31 38 45 



p ... 31 28 22 16 12 



def. force. 



real. 45 



lexp. 44 44 40 35 30 



The perfection of the agreement here emphasises the distinct- 

 ness of the exceptional behaviour at the higher pressures. 



Jn the case of hydrogen we have at the higher pressures 

 the following values, the numbers in the third row being the 

 products of repulsion by pressure, which ought according 

 to (27) to be tending to a fixed limit : — 



p 1000 921 526 421 330 



def. force 113 4 5 



1000 921 1578 1684 1650 



p... 314 234 205 179 



def. force 8 11 14 18 



2512 2574 2870 3222 



The products do not show the same approach to a limit as was 

 the case with air, and there is a jerkiness in their variation 

 which points probably to experimental uncertainty. At the 

 lower pressures the quotients of repulsion by pressure, which 

 ought to be tending to a limit, are given in the third row of 

 the following : — 



p 16 14-5 12 8 6554 



def. force 52 49 45 37 31 29 26 



32 3-4 37 4-6 48 58 6*5 



Here again the convergence to a limit is not satisfactory, a 

 state of affairs which will be traced in the separate paper on 

 measurement of low pressures to inaccurate values of the 

 lower pressures ; and in that paper it will be shown that 

 hydrogen exhibits a peculiarity which expresses itself in our 

 equation (27) by dividing A" by (1 — up) where a is another 

 parameter, thus for hydrogen the deflecting force is 



<//{A"p/(l-ap) + B'" + l/p}, . . (28) 



which makes the deflecting force when p= ]/a, a result to be 

 extended to all higher pressures ; the values of the parameters 

 are A"='0006, «='0016, B'" = -01, and c'=4'16, which 

 give the following comparison : — 



