EXPERIMENT STATION BULLETINS. 617 



tural soils and why a similar diminution does not occur in the more 

 artificial substances. It mu.st be admitted, however, that for this 

 phenomenon also, no definite and proved explanation can be offered. 

 The following suggestions, however, seems to deserve and warrant con- 

 sideration. 



It was first thought that the soil solution at the low moisture might 

 have reached its saturation point in concentration and as it cooled, 

 salts separated, for the solubility of the salts diminishes with falling 

 temperature, and thus the solution became less concentrated. This 

 hypothesis, however, would not explain the continued decrease in de- 

 pression with repeated freezings, for if the soil solution were concen- 

 trated to the saturation point at the low moisture content all the excess 

 of salts should separate the first time that it is cooled below 0°C, and 

 inasmuch as probably not all of the separated salts would go again 

 into solution upon thawing — owing to the short length of time — the 

 concentration of the solution and hence the lowering of the freezing 

 point should remain more or less stationary. Hence, this hypothesis 

 fails to explain the results. 



If it is true that the soil adsorbs water and that this adsorbed water 

 is removed from the liquid phase and becomes almost solid like water 

 of crystallization and it no longer acts as a solvent of the salts in the 

 soil, then the foregoing phenomenon on the diminution of the depression 

 with repeated freezings might be explained under the following hypo- 

 thesis: The adsorption of the water is due, at least in part, to the 

 colloids in the soil. Upon freezing, these colloids coagulate and the 

 bonds uniting them with the water break and the adsorbed water be- 

 comes liberated. This liberated adsorbed water becomes available and 

 goes to dilute the original solution and thereby decreases the lowering 

 of the freezing point. Thus, for instance, if there were only 5% of 

 available moisture before the first freezing was made, there is probably 

 6% at the end of the first freezing, 7% at the end of the second freez- 

 ing, and so on, until probably all or most of the colloids are coagulat- 

 ed. The coagulation is not complete in the first freezing but the great- 

 est amount takes place in the first freezing and then smaller amounts in 

 the successive freezings. 



This hypothesis seems to find a large amount of confirmation from 

 both direct and indirect evidences. It is now generally believed that 

 there are present in soils both hydrosols and hydrogels consist of 

 organic substances, the humus compounds, and inorganic substances, 

 the hydrates of alumina, iron, manganese, hydra ted silica, the zeolitic 

 silicates, etc., that the hydrosols are continually being produced in the 

 soils by physical action such as that of the water on clay, or by chemical 

 changes such as the decomposition of silicates; and, that there are in 

 nature various agencies such as changes in temperature, concentration 

 of solution, etc., by which hydrosols are changed to hydrogels, and vice 

 versa. Changes in temperature are known to produce especially very 

 marked effects upon these colloids. Both heating and freezing have 

 been found to cause coagulation of the hydrosols and hydrogels and 

 that this coagulation is accompanied by a decrease in the absorbtive 



