CO-ORDINATION OF DALTON'S, GRAHAM'S, AND MITCHELL'S RESULTS. 53 



diate object of this paper. I have already stated that the results of Dr. Mitchell and 

 Professor Graham apparently exhibit a striking discordance ; it will here be seen that 

 the facts reported by those chemists can be readily co-ordinated. 



166. Both of them appear to have made trials of the absorbent power of the barriers 

 thev respectively employed ; Professor Graham having operated on a mass of stucco 

 of certain dimensions, and found its absorbing power, in relation to most gases, very 

 low ; Dr. Mitchell on a thick cylinder of gum elastic ; but neither of them appears to 

 have clearly seen the importance of this element in the production of the final result. 

 In the case of the action of stucco, this, indeed, is a remarkable circumstance, for in all 

 those instances where the absorbing power of the stucco was great, the equivalent vol- 

 umes of diffusion, as obtained, were, without exception, erroneous. Dr. Mitchell, on 

 seeing certain gases pass into each other with a force that was greater than the pres- 

 sure of sixty-three inches of mercury, and inferring that there was no vis a tergo in 

 play, was obliged to impute his result to the inherent power of gaseous penetration; 

 hence he came into direct collision with the Daltoniaii hypothesis. On the other hand, 

 Professor Graham, supposing that, in all his erroneous cases, the deficit was to be im- 

 puted to the porous mass, which, in some manner, detained and absorbed the gases, 

 found in every other instance a full confirmation of the doctrine of a vacuum. 



167. The whole phenomenon depends, however, upon the action or inactivity of 

 the cellular tissue itself; it will be convenient, for the better understanding of it, to con- 

 sider it under two heads. First, where the tissue exerts no absorbent action on the 

 media, or absorbs both to the same extent ; and, secondly, where one is absorbed to a 

 much greater extent than the other. 



168. In the first case, the velocities with which any two gases pass into a vacuum are 

 inversely proportional to the square roots of their densities respectively ; moreover, the 

 volumes that so pass vary directly as the velocities, and therefore may be taken as an 

 index and measure of them; but, as the mass of each gas is expressed by the product 

 of its density into its volume, it may be also represented by the velocity multiplied into 

 the density ; and, as the square of the velocity of the one, multiplied into its density, is 

 equal to the square of the velocity of the other multiplied into its density, whatever may 

 be the difference of the specific gravity of the two gases, their mechanical momentum 

 will always be the same; the resistance they meet with in passing through the tissue is 

 common to both, and equal in both cases ; and hence the initial velocities of diffusion 

 ought to be inversely proportional to the square roots of the densities ; and as, during 

 the progress of the experiment, the impelling force of the one gas is equal to the ex- 

 pelling force of the other, the resulting momenta of the two currents is still equal, and 

 the final volumes are such as are found by direct experiment. 



169. We now come to consider the second case, where the cellular tissue presents 

 one of the gases in a condensed form to the other, or, in other words, absorbs it ; and 

 here we have to refer to a fundamental proposition of dynamics, that when the moving 

 force and the matter to be moved vary in the same proportion, the resulting velocity 

 will always be the same. An illustration will show the application of this principle to 

 the case in hand. If a cylinder of air, fitted appropriately with a piston, communicates 



