LIQUIDS AND ALLIED EXPERIMENTS. 35 



The case of hydrogen, diffusing into air, does not show large values in m; 

 this might seem to be due to an even more rapid diffusion during the first 

 day, decaying more rapidly. It is, however, at once interpreted on reduc- 

 ing m to v, the diffusion relative to volume (see table 9). 



The initial diffusion of hydrogen out of the swimmer into air, and of 

 hydrogen into the swimmer containing air, is in excess of the opposed 

 current of air, by an amount of about the same order. The rate at which 

 imprisoned hydrogen escapes from the swimmer in excess of the entrance 

 of air is, however, definitely larger than the excess of rate of entry, compared 

 with the escape of air in the converse case, for reasons which do not at once 

 appear. These rates are v X io 6 = 8 to 10 for escaping hydrogen, and 

 dXio 6 = 5 for entering hydrogen, and are to be somewhat modified by the 

 constants of the apparatus. Possibly the fact that the water must first be 

 saturated with hydrogen before this gas can enter the swimmer, whereas it 

 escapes with greater freedom into unsaturated water, accounts for this initial 

 difference. At the same time, hydrogen enters the swimmer against the 

 pressure gradient of the water levels. 



In general, however, the diffusion of mixed gases, during the first day, 

 is in need of more detailed investigation, in which case special methods of 

 charging and of observing without removing the artificial atmosphere, will 

 have to be devised, as is done in the last chapter. 



The special feature of table 9 is the relatively low rate of the final diffu- 

 sion of mixed gases. Thus the mixture air-hydrogen (table 8), which is 

 nearly all air, shows a lower rate than pure air; the final hydrogen-air 

 diffusion, which is nearly all hydrogen, a lower rate than pure hydrogen; 

 the final hydrogen-oxygen diffusion, a much lower rate than pure hydrogen ; 

 the hydrogen-oxygen-nitrogen diffusion, which is nearly all hydrogen, a 

 much lower rate than air or hydrogen, etc. It seems, therefore, that small 

 quantities of a second gas added to the pure gas markedly reduce its rate 

 of diffusion, k. The reason of this would seem to be the potential energy 

 of the mixture, which must here be increased to the potential energy of 

 separated gases. Thus the opportunity of dissipating potential energy is 

 decreased and the diffusion is correspondingly sluggish. 



It is, however, probably sufficient to refer the whole question to the actual 

 value of the full pressure gradients involved. If p a , p b , p e , . . . , are the 

 partial pressures of the gases within the swimmer, when B is the barometric 

 pressure and h" p^g the pressure due to the heads of water bearing on the 

 gas mixture and t the vapor pressure, 



Pa+h+Pc+ ' ' ' =B-T + h" Pw g 



Hence if the pressure B — t is due to the gas a (artificial atmosphere), it 

 will enter the swimmer if 



P*+Pc+ ' ' ' >V'p»g 



it can not escape from the swimmer until p b -\-p c + ' ' ' = h"p v g. As this 

 is a small pressure, the gases b, c, . . . must themselves escape slowly, 



