84 



THE DIFFUSION OF GASES THROUGH 



Table 45 shows the results as thus far obtained. 



To compute k, the coefficient of volume diffusion, the equation becomes 



K = v/a (dp/dl) = h{i + 2h"'/h") Ipg 



where h is the rise per second of the lower meniscus and the fall per second 

 of the upper, //'the head of liquid of density p, 2h"'-rh" the total length of 

 column through which diffusion takes place. The table shows that on 

 the average // = 26. 45 — 26.37=0.08 cm. below, and 16.27 — 16.20 = 0.07 cm. 

 above, in about 17^ months, or 45.6 X lo"^ seconds. Hence 



/ 0-075 ,., ,2h"' 

 h = ~T^. — 6 while I + 



46X10" 



h' 



9.4 

 I + 77-^ and pg = 98 1 



10.2 



Therefore k = o.03X io"^*^ nearly, a value, even in consideration of the long 

 time of observation, 17 months, and the small displacement of meniscus, 

 surprisingly below the datum furnished by experiments with the Cartesian 

 diver above, k = 0.8X10"^". 



T.\BLE 45. — Diffusion in U-tube. 



This experiment shows that, so far as the manometer is concerned, the 

 diffusion error will be negligible. It is by no means so, however, when it 

 is a question of storing a pure gas over water in air. 



Nothing, however, has been brought forward to suggest why this direct 

 result for k with the manometer should be but 4 per cent of the value found 

 by the diver, unless it be the continual or intermittent churning up of 

 gradients by temperature, and by the diver in the latter case of wide tubes, 

 as compared with the fixed gradients in the narrow tube of the manometer. 

 The disparity of values is one which can only be settled in the lapse of much 

 more time, inasmuch as h is as yet too small to be trustworthy. If it takes 

 the divers nearly a month to reach equilibrium conditions, it should take 

 the manometer much longer, and the experience with the long divers in 

 §41 may be recalled. Any small difference in the gas above the two 

 meniscuses of the manometer, produced, for instance, in closing the tube 

 with the blow-pipe, would be a serious consideration in case of the small 

 amount of gas in either shank of the U-tube. Correlative experiments 

 with wider tubes naturally suggest themselves, and the same have been 

 installed to be read off next year. 



