508 Lord BIythswood and Mr. H. S. Allen on 



neglected. Consequently the relation between the pressure 

 and the time must be expressed by the more general equa- 

 tion (iii.). This was found to agree well with the experi- 

 mental results. The values of X are given in the last column 

 of the Table. 



§ 18. It will be observed that the rate of absorption (as 

 measured by X) is about eight times as large in the earlier 

 portion of the experiment as it is in the later part, when the 

 value is deduced from the readings of the McLeod gauge. If 

 we compare the pressures under which the absorption is taking 

 place, we find an immensely greater difference. In the first 

 part of the experiment, the pressure is of the order of 200 mm., 

 in the second part it averages only '04 mm., so that while 

 the ratio of the pressures is of the order of 5000 to 1, the 

 ratio of the corresponding values of X is only about 8 to 1. 

 This result emphasizes the conclusion arrived at from the 

 experiments formerly described, that the rate at which ab- 

 sorption of gas takes place is influenced to only a very slight 

 extent by the pressure of the gas. 



§ 19. It must be remembered that in the experiments de- 

 scribed we are dealing with a mixture of gases, so that the 

 results are more complicated than they would be if we had to 

 do with a pure gas. Dewar* has shown that one cubic centi- 

 metre of cocoanut charcoal can absorb 155 c.c. of nitrogen, 

 or 230 c.c. of oxygen at a temperature of —185° C. Con- 

 sequently, in the present set of experiments the percentage of 

 nitrogen in the unabsorbed gas will increase as the absorption 

 proceeds, so that the velocity coefficient deduced will not be 

 constant. Both the alteration in the composition of tlie 

 gaseous mixture and the effect of diminution of pressure on 

 the velocity, will tend to give a smaller coefficient of velocity 

 as the experiment advances. 



§ 20. Three separate experiments were carried out with the 

 large charcoal receptacle already referred to (§ 5) sealed to 

 the McLeod gauge. By means of a Fleuss pump the whole 

 apparatus was exhausted until the mercury stood at 740 mm., 

 the barometric height being 780 mm. After the charcoal 

 had been cooled to the temperature of liquid air, the pressure 

 in the apparatus fell for over three hours ; the final pressure 

 registered being '0009 mm. The readings of the McLeod 

 gauge obtained in experiments on three consecutive days 

 showed remarkable agreement, and it has been thought worth 

 while to tabulate these not only to illustrate the regularity of 

 the absorption, but also to strengthen the confidence that may 

 be placed in the readings of the gauge (Table VI.). 



* Proc. Roy. Soc. lxxiv. p. 121 (1904), 



