J. A. Prescott 125 



brought into the solution. Adsorption proceeds normally in the 

 presence of a second substance but the constants of the isotherm are 

 changed. Masius^, Michaelis and Rona^ and Schmidt have shown 

 that if two substances are adsorbed the adsorption of each one is 

 diminished but it can still be expressed by the usual equation. 



Van Bemmelen discusses the question whether absorption by gels 

 is of the same nature as ordinary adsorption by charcoal, silica, etc. 

 A web structure is attributed to the gelatinous precipitates and he 

 points out that there is no essential difference in these cases between 

 adsorption and absorption but he prefers to use the term " absorption" 

 as being more general. In support of this view those precipitates which 

 were supposed to have the largest "web" surface showed the highest 

 adsorptive power and also those precipitates which after ignition were 

 capable of becoming incandescent also showed very active adsorption. 

 The gels used by Van Bemmelen must not be regarded as jellies in the 

 usually accepted sense of the term. One of his silica gels is described 

 as "a dry dusty powder" and most of his absorbents must have been 

 in this form. They can only be described as colloids in that they 

 possessed a structure in which the discontinuities were of colloidal 

 dimensions, that is between 0-1 /x and 1 /ajli. Adsorption is not a 

 property peculiar to colloids although colloids show this property to 

 a remarkable extent on account of the large surface they possess in 

 proportion to the total mass. 



Applications to soils. 



Side by side with all this work, agricultural chemists have been 

 trying to apply the above laws to the known phenomena of absorption 

 by soils. The simplest type of absorption by a soil is shown by a dye- 

 stuff. Here there is no chemical reaction but every probability that the 

 phenomenon is purely physical. The adsorption of dyes by soils is 

 found to follow the usual laws. H. E. Patten and F. K. Cameron^ 

 found a definite equilibrium with gentian violet, eosin and a manure 

 extract which, when plotted as a curve, was seen to be of the usual 

 adsorption type. Although they recognised the value of the adsorption 

 equation for more simple cases, they did not attempt to express their 

 soil curves by this equation as they considered the soil conditions were 

 too complex. They concluded however that the distribution of solute 



1 Dissertation, Leipzig, 1908. - Biockem Zeits. 1908, 15, 196. 



^ Jouni. of Phys. Chemistry, 1907, 11, 581. 



