Mr. T. Graham on Liquid Diffusion applied to Analysis. 211 



The superimposed column of water being 111 millimetres 

 (4*38 inches) in height, the chloride of sodium, it will be observed, 

 lias diffused in sensible quantity to the top, and could have risen 

 higher; the upper layer being found to contain 0*104 gramme 

 of salt, or 1 per cent, of the whole quantity present. The apex 

 of the diffusion column of sugar appears to have just reached the 

 top of the liquid in the fourteen days of the experiment, for '005 

 gramme only of that substance is found in the first stratum, 

 followed by -008, -012, -016, and -030 in the following strata. 

 Again, no gum appears to be carried by diffusion higher than 

 the seventh stratum (2*2 inches), which stratum contains '006 

 gramme, followed by *031 gramme in the eighth stratum. The 

 minute quantities of substance shown in the first to the sixth 

 stratum, and which do not altogether exceed *020 gramme, are 

 no doubt the result of accidental dispersion, arising probably 

 from a movement of the upper fluid occasioned by slight inequali- 

 ties of temperature. The diffusion of tannin is even less advanced 

 than that of gum ; but the former numbers are apparently in- 

 fluenced by a partial decomposition, to which tannin is known to 

 be liable, and which gives rise to new and more highly diffusible 

 substances. 



Experiments continued, like those last described, for a con- 

 stant time, do not exhibit the exact relative diffusibilities, although 

 these could be obtained by proceeding to ascertain, by repeated 

 trial, the various times required to bring about a similar distri- 

 bution and equal amount of diffusion in all the salts. The 

 numbers observed, however, may afford data for the deduction 

 of the relative diffusibilities by calculation. 



A particular advantage of the new method is the means which 

 it affords of ascertaining the absolute rate or velocity of diffusion. 

 It becomes possible to state the distance which a salt travels per 

 second in terms of the metre. It is easy to see that such a 

 constant must enter into all the chronic phenomena of physiology, 

 and that it holds a place in vital science not unlike the time of 

 the falling of heavy bodies in the physics of gravitation. It may 

 therefore be not amiss to place here in a short tabular form the 

 results observed of the diffusion of a few more substances, con- 

 ducted in the same manner as the preceding. 



P2 



