488 A. B. BEAUMONT 
Hilgard (1911) observed that his ‘‘colloidal clay,” which could be 
obtained from a suspension either by evaporation or by flocculation, 
exhibited reversibility. When it dried it shrank as so much boiled starch, 
and on being remoistened it swelled quickly, resuming its former jelly-like 
consistency. Moistened with less water it became highly plastic and 
adhesive. 
Grout (1906), in attempting to raise the plasticity of clays, found that 
agar-agar mixed with clay was effective but alumina cream was less 
effective. Furthermore, after being air-dried, powdered, and mixed, the 
plasticity of the alumina cream as used was irreversible. Grout also 
prepared an artificial hydrated silicate of alumina, and found that as 
also was irreversible after drying. 
Ashley (1909), in commenting on Grout’s work, said it seemed to Grout 
that none of these or other colloids could be responsible for the behavior 
of natural clays; for he apparently thought that clay could be dried and 
wetted repeatedly without injuriously affecting its plasticity, or, in other 
words, without affecting the activity of the colloids. Ashley said further 
that in this supposition Grout was not in accord with practical experience. 
Clay used by potters has in most cases never been deprived of its natural 
moisture. After once being dried out at as low a temperature as 60° C., 
it is found to have lost noticeably in plasticity. 
Ehrenberg (1915) cites the work of Thaer and Ostwald, and states that 
it can be said with a considerable degree of certainty that humus is revers- 
ible. How far humus is affected by aging is not known. The study of it 
is restricted by its not having a definite chemical composition. 
Little is known concerning the behavior of humus gel, according to 
Ehrenberg, but he states that according to Zailer and Wilk humus gel 
does not suffer many changes in drying out; that it exhibits simple swelling 
phenomena, but does not come back to its original volume if it is more 
or less dried. To substantiate this poimt he cites data from Wollny 
showing that the greater the depth at which dried peat was taken, the 
more it increased in volume on being moistened. The surface layers, 
having been dried the more, increased less. 
Warington (1900) cites Schloesing’s experiment in which he mixed 
calcium humate with clay and found that the humate had considerable 
cementing power, which it lost on being dried. The irreversible character 
here is probably due to the absorption of the calcium in the preparation 
