354 Mr. J. J. Hood on the Rate of 



the evolution of aqueous vapour, which would happen with a 

 weak solution. It was also necessary to maintain the volume 

 of the absorption-vessel constant, so that the pressure of the 

 unabsorbable gas should not vary, otherwise there would be 

 introduced an unknown factor — that relating to the interdif- 

 fusion of one gas through another at varying pressures ; then, 

 by employing a reasonable hypothesis regarding the rate of 

 absorption of a gas, to obtain a formula which with the expe- 

 rimental numbers might form the basis for the calculation of 

 the rate of interdiffusion of the two gases. 



This hypothesis, that the rate of absorption is proportional 

 to the pressure, has already been employed by Hurter* in his 

 experiments on the absorption of chlorine and carbonic dioxide 

 by lime, viewed from a technological point, and was supposed 

 also by Graham to be true for the rate of passage of a gas 

 through a porous diaphragm. The method, however, that the 

 former chemist employed does not admit of that desirable 

 accuracy requisite to establish the hypothesis, and indeed the 

 experiments in the present paper do not tend to confirm its 

 truth. 



The absorption-vessel employed had a capacity of 136'8 

 cubic centim., or, with the connecting-tubes forming part of 

 the gas-space, of 138*2 cubic centim. The capacity of the 

 500 millim. of the manometer-gauge was *845 cubic centim., 

 so that between the start and the finish of an experiment the 

 volume varied only by about # 6 per cent.; thus practically the 

 pressure of the unabsorbable gas was constant. The initial 

 pressure of the gas to be absorbed that was introduced into 

 this vessel was equal to 500 millim. of water, or about 6*7 

 cubic, centim., forming 4*82 per cent, of the whole volume. 

 10 cubic centim. of the potash solution (the volume that was 

 employed) contained 3*54 grms. KHO, or equal to about 105 

 times the amount requisite to combine with all the gas. 



In order to fulfil the requisite conditions of maintaining the 

 volume of the absorption-vessel constant, introducing any 

 desired quantity of the gas to be absorbed, and measuring the 

 variations of pressure during an experiment, the following 

 apparatus was employed, as shown on Plate X. fig. 4. 



The absorption-vessel, A, measured from the surface of the 

 potash to the top 195 millim., and had an internal diameter of 

 28'6 millim., determined by calibration with mercury. The 

 side chamber /for the potash solution had two marks etched 

 on it, the intervening volume being 10 cubic centim. ; the 

 glass stopper a, ground into the bottom of/ and air-tight, was 



* Moniteur Scienti/lque, 1878. 



