Wild and Anderson. — Absorption of Lime by Soils. 



475 



These results are shown graphically in fig. 6, (a) and (b). 



Tune uv Hours 



Fig. 6. 



It will he noticed that the form of the two curves given is almost identical ; 

 and, further, that the time taken to complete a certain fraction of the 

 reaction appears, within the limits of the experimental error, to he inde- 

 pendent of the initial concentration. For instance, the time taken to com- 

 plete one-third of the reaction is approximately half an hour in both cases. 

 Now, this is characteristic of reactions of the first order (Ostwald)— i.e., 

 reactions where only one substance undergoes any alteration. Many col- 

 loidal reactions are of this type, and it may be that here the soil colloid 

 remains of constant activity and that the course of the reaction is con- 

 ditioned solely by the concentration of the calcium hydroxide. 



General Conclusions. 



The results recorded in this paper appear to us to indicate conclusively 

 that the interaction between soil and solution of calcium bicarbonate or 

 hydrate is a colloidal phenomenon. If this is the case, then it follows that 

 a method of determining the lime requirement of a soil based on this (such 

 as the Hutchinson-MacLennan method) is entirely without scientific founda- 

 tion. The Hutchinson-MacLennan method, however, taken merely as an 

 empirical method and operated under standard conditions, promises to be 

 of great utility, as has already been shown in the first part of this paper. 

 It is certainly better than any method based on chemical analysis, and 

 compared with other methods of a similar kind hitherto used it has the 

 advantage of ease of manipulation. 



