Chemical Absorption of Gases. 353 



If in a closed vessel containing a homogeneous mixture of, 

 say, two gases, it were possible so to arrange matters that, of 

 the molecules of the two gases which impinged on a fixed area 

 of the side of the vessel, only those of one sort passed out into 

 a vacuum without any returning, the rate at which the vessel 

 would be emptied of this gas would give a measure of its rate 

 of interdiffusion through the molecules of the other gas. To 

 effect such an ideal process is doubtless an impossibility ; but 

 the practical method, imperfect it may be, that offers itself for 

 accomplishing this operation, that of selective absorption by 

 chemical means, forms the subject of the present investigation. 



The objections that can be raised against this method for 

 studying the rate of interdiffusion of gases are great, and these 

 are in part considered experimentally at the end of the paper; 

 still the results that have been obtained may be of some 

 interest. For instance, it is shown that, with a given absorb- 

 able gas, the rate at which it is absorbed is less when it is 

 mixed with air than when mixed with the lighter gas, hydro- 

 gen : this is especially noticeable in the first stages of the 

 absorption. On the other hand, however, when the atmo- 

 sphere is the same, the rate of absorption in relation to the 

 density of the gas does not come out in a satisfactory way. 

 Of the two gases C0 2 and SH 2 , for part of the course a marked 

 difference in their rates of absorption is visible, the lighter 

 gas SH 2 being absorbed more rapidly. This is especially 

 noticeable in an atmosphere of hydrogen ; the same is true 

 for the gases Gl and S0 2 . These facts, which are principally 

 observable in the first stages of the experiments, are in har- 

 mony with previous experimental work on gaseous diffusion ; 

 but a strange anomaly is brought to light, and it is this — that, 

 although there is a difference in the rates of absorption of the 

 gases C0 2 and SH 2 in favour of the lighter one, and also of 

 the two gases CI and S0 2 , these latter, each possessing a higher 

 density than the two former gases, are absorbed with much 

 greater rapidity than either C0 2 or SH 2 , especially when an 

 atmosphere of air is employed ; when hydrogen is used, SH 2 

 and CI are absorbed with nearly equal rapidity. 



In the method here adopted for getting out these facts, the 

 perturbations that might arise from physical causes at the 

 surface of the absorbent were attempted to be overcome, or at 

 least lessened, by employing a comparatively small volume of 

 the absorbable gas diffused through a large volume of another 

 gas — air or hydrogen — and using a strong solution of potash 

 so as to have it very much in excess over that required for 

 complete absorption of the gas. Strong potash solution was 

 employed also in order to prevent complicating matters by 



