130 Prof. A. J. Brown and Mr. F. Tinker. Absorption of 



progressively greater extent the forward diffusion pressure of the solution 

 outside. The rate of absorption of the solution is at a maximum when the 

 seeds are absolutely dry, because the backward pressure tending to make the 

 solution diffuse out again is then zero. It is zero at equilibrium because the 

 backward diffusion pressure has then become equal to the forward diffusion 

 pressure of the solution outside. 



(b) affect of the Nature of the Solution, etc., on the Absorption Bate : the 

 Absorption Constant. — Tt has been indicated already that, in order to compare 

 the relative rates at which various solutions are absorbed by the seeds, equal 

 degrees of fulness x should be chosen. But if x is given a constant value for 

 all the solutions in any series, the value cc—x for all the solutions will be the 

 same also, since the magnitude of the equilibrium weight a is practically the 

 same in every case (see Table VII). It would follow, therefore, that the 

 relative rates of absorption of the various solutions at any given degree of 

 fulness (a;) are proportional to their constants k. The constant k might hence 

 be termed the absorption constant for the given solution. It gives a measure 

 of the relative rate at which the solution will enter the seeds at all degrees of 

 fulness. Thus, whatever weight the seeds may have attained, seminormal 

 phenol always enters the seeds with about twice the velocity that water 

 enters, since its absorption constant is about twice that of water. 



An inspection of Table VI shows that the absorption constants of all the 

 phenolic solutions are greater than that of water, with the exception of 

 pyrogallol, which has a constant practically the same as that of water. The 

 constant increases greatly in value with a slight rise in temperature. This 

 increase is an exponential one rather than linear, being like vapour pressure 

 in this respect. Indeed it has been shown* that the curves showing the 

 relationship between the rate of entry of water into the seeds and the 

 temperature can almost be superposed on the vapour pressure-temperature 

 curves for water, either being represented by the equation 



v = ce* 9 , 



where v represents either the velocity of absorption of water or the vapour 

 pressure, and 6 the temperature, k and c being constants. 



Since it has now been shown that the rate of absorption is always propor- 

 tional to the absorption constant, it follows that the latter is an exponential 

 function of the temperature, i.e. 



k oc ce* 9 , 



where k has the same value as in the temperature-vapour pressure curves. 

 In other words, if a series of temperatures be chosen, the absorption 



* A. J. Brown and F. P. Worley (loc. cit.). 



