78 REPORTS ON THE STATE OF SCIENCE.—1918. 
physical effects. On the other hand, slightly acid substances 
such as superphosphate have no bad effects, but rather the reverse. 
Clay is thus an electro-negative colloid, its reaction probably 
being conditioned by a trace of potash liberated by hydrolysis. 
It shows the general properties of electro-negative colloids as 
elucidated by Schulze and by Hardy: thus it is flocculated only 
by a solution containing ions or particles of opposite electrical 
sign, and the extent of flocculation increases rapidly with the 
valency and concentration of the ion. No quantitative relation- 
ships, however, could be found by Hall and Morison. 
Pickering throws over the electrical hypothesis and attributes 
flocculation to a combination of the clay with the flocculant and 
the solute, whereby the aggregation of molecules increases so 
much that it loses its power of Brownian movement and soon 
settles, 
Other effects of Colloids. 
Various obscure changes are brought about on drying the soil. 
There is a marked increase in the proportion of water-soluble 
material, and, as Buddin has shown, in the ease with which 
nitrates are formed. Changes in the micro-organic population no 
doubt account for some, but not all, of these effects, and the 
simplest explanation is to attribute them to the changes in the 
colloids. Again, soil has a remarkable power of decomposing 
hydrogen peroxide, which is affected by small quantities of 
various substances. and may be a colloidal phenomenon. ‘The 
een of cyanamide in the soil has been attributed to 
colloids. 
The influence of Colloids on Bacterial and Plant Lnfe. 
Plants and bacteria draw from the soil their water and their 
nutrient salts, and are therefore profoundly affected by anything 
that retards delivery of these essential substances. Inasmuch as 
colloids possess markedly absorbent powers they might serve as 
reservoirs to protect against loss by evaporation or leaching, or, 
on the other hand, they might actually compete against the plant 
and hold some of the supplies the plant ought to obtain. 
As yet there is insufficient experimental evidence to show how 
the various factors are likely to interact. Fortunately ecologists, 
both in this country and elsewhere, are fully alive to the possi- 
bilities: Gola has discussed the influence of the colloidal complex 
in determining plant habitats, and Séhngen its effects on the 
activities of micro-organisms; an illuminating résumé by Cavers 
has also appeared. It is interesting to note that soils destitute 
of colloidal properties are often infertile. 
The Estimation of Soil Colloids. 
Many efforts have been made to estimate the amount of colloidal 
matter in the soil. They fall into two groups based on the 
absorption of dye stuffs and of water vapour respectively. 
Methods based on the absorption of dye-stuffs have been elaborated 
