324 



270 



■616 



•995 



8-9 



8-0 



9-3 



8-2 



94 Mr. W. Sutherland on two New 



which, under ordinary circumstances, ought to have the 

 constant value 2a\ p /~D. 



p by gauge... 330 234 155 110 59 37 26-5 20 14-5 8 



10 4 log. dec. . 495 488 484 472 441 408 373 351 



•009 -024 -034 -063 -145 '247 -378 -476 



3-0 5-6 5-3 69 8-6 9"0 10-0 9-5 



12-9 12-6 9-9 102 10*3 102 10-8 10'1 



The numbers in the fourth row instead of being constant rise 

 to a maximum and decline; but if to the numbers in the 

 third row, which represent 2£/D, we add '03 and then multiply 

 by p, we get the products given in the last row, which are 

 practically constant down to 20/10 6 atmo. Thus, then, the 

 free path near the solid X s , which is proportional to the co- 

 efficient of slipping, instead of being proportional to X is 

 proportional to X— /*, where h is a constant. 



Now in the formula for deflecting force A /f p is the form 

 given to a term varying inversely as X s , so that this term 

 cannot be written as inversely proportional to X, and there- 

 fore directly proportional to p, but it must be taken as 

 inversely proportional to X(l— /*/X), that is to (1— ph/\QZ) )/p 9 

 and therefore the term from which A! r p is derived in the 

 general theory gives A^pftl—pJi/Xopo) in the case of hydro- 

 gen, which, in " Thermal Transpiration and Radiometer 

 Motion,'"' we actually found to be the case, if we identify 

 Ji/XoPo with the constant a of that paper. And there we also 

 found the value of A" to take its theoretical place along with 

 those for other gases, which shows that in connexion with 

 the deflecting force X s in hydrogen is altered in its relation 

 to X simply by a factor such as (l—ph/\ Q p ) with a limit 1 

 and not any other number ; but if from the last table we 

 take 10 as the value of (2£/D + m 03)p, and compare it with 

 the value 15 for (2%/D)p in air, which stands for 2aX p /T), 

 and remember that at any standard pressure p the values of 

 X for hydrogen and air are as those of -n'/mi, that is nearly as 

 2 to 1, we find that (2£/~D + '03)p for hydrogen would have 

 to become 30 to give this theoretical relation of 2 to 1 if the 

 alteration of *03 were all that is required ; this then indicates 

 that with hydrogen, in addition to a factor (1 —ph/X p ) in 

 the relation of X s to X, there is a factor 1/3 whose occurrence 

 here, and not in deflecting force, has to be explained. 



The explanation probably lies in the presence of a trace of 

 water-vapour, for notwithstanding the very thorough mea- 

 sures which Crookes took to get rid of the water-vapour 

 after it had given him considerable trouble, I am still in- 



