Chemical Absorption of Gases. 365 



most rapidly or in which the greater portion of the gas is ab- 

 sorbed in the first two or three minutes. This might be due 

 to the salt produced by the gas and the potash forming a kind 

 of skin on the surface of the absorbent, and so retarding further 

 action. If such were the case, and if the ratio of the pres- 

 sures at equal intervals of time ought in truth to be constant, 

 by making experiments in which the gas had initially dif- 

 ferent pressures the values of this ratio would be somewhat 

 similar. To investigate this point, the gas S0 2 was selected 

 in an atmosphere of dry air, as here this coefficient varies 

 considerably between limits of pressure easily accessible to 

 experiment. From a number of experiments the following 

 three (p. 366) are selected, and they show very clearly that 

 this coefficient really does increase as the pressure diminishes. 

 This result may in fact be an indication that the coefficient of 

 diffusion of a gas is a quantity variable with the pressure ; 

 which has not yet been shown satisfactorily in an expe- 

 rimental way, although theoretical considerations seem to 

 suggest the idea. 



It seems scarcely possible that the very much greater rate of 

 absorption of the heavier gases CI and S0 2 than that of C0 2 , 

 under like circumstances, can be due to the formation of cur- 

 rents or a down-pouring of the gases upon the surface of the 

 potash. In order to see if such could be the cause of the 

 anomaly, several experiments were made in the following 

 way: — The absorption-vessel was charged in the usual manner 

 with C0 2 in an atmosphere of hydrogen and allowed to stand 

 twenty hours, the solution of potash run in, and the rate of 

 absorption observed. When the experiment was completed 

 and all the gas seemed to be absorbed, C0 2 was allowed to 

 flow into the apparatus upon the potash until the manometer 

 fell to zero, the inlet of the gas immediately closed, and the 

 gauge observed as before. In this case, the C0 2 being twenty- 

 two times heavier than the atmosphere (hydrogen), the former 

 gas might be expected to tumble down to the bottom of the 

 vessel more or less in the form of a cloud, and its rapid ab- 

 sorption indicate the fact. Such, however, did not seem to 

 be the case. The water in the manometer began at once to 

 rise, and during the first five minutes, or till about 65 per 

 cent, of the gas w r as absorbed, the rate of absorption was con- 

 siderably slower than when the gases had been allowed to 

 mix thoroughly for twenty hours. After the fifth minute the 

 rate began to increase slightly, and continued to be greater 

 than in the first experiment, the pressure of the C0 2 being 

 also greater up to the end of the thirtieth minute; by the end 

 of the sixtieth minute the manometric reading was still a few 



