480 A. B. BEAUMONT 
lem has been attacked in many ways previously. It was thought well to 
attack it from the standpoint and with the methods of colloid chemistry. 
It has resolved itself into a study of the reversibility of the colloidal condi- 
tion of soils. 
REVERSIBILITY DEFINED 
Severai different meanings have been attached to the word reversibility. 
According to Ostwald (1915 : 40), ‘“‘when a change in the state of a colloid 
may be\reversed by reversing the conditions which brought that change 
about, it is said to be ‘reversible.’’? The word is commonly applied to 
the change between the sol and gel conditions. Thus, if a colloid which 
has been precipitated by a salt goes back into solution on removal of the 
salt by washing, the colloid change is said to be reversible; if this does 
not occur, the colloid change is irreversible. 
Ostwald states that the reversibility of a change is not determined, in 
the main, by the nature of the colloid itself, but by the character of the 
conditions that produce the coagulation. For instance, the precipitation 
of the typical protein sols by neutral salts is reversible, whereas their 
precipitation by heat is irreversible. Ostwald states that one cannot, 
therefore, properly speak of reversible and irreversible colloids, but only 
of reversible and irreversible changes. 
Zsigmondy (Zsigmondy and Spear, 1917) divides colloids into reversible 
and irreversible, depending on whether or not they leave a soluble residue 
on evaporation. The irreversible colloids he still further divides into 
two groups: to the first class belong those that coagulate in dilute solu- 
tion and precipitate in the form of a powder rather than as jelly; the 
second class consists of those that may be considerably concentrated 
before coagulation sets in, and whose precipitates are decidedly jelly-like, 
such as colloidal silicic acid, clay, and ferric oxide. The colloids of the 
first group are considered completely irreversible. Those of the second 
group, which includes some most commonly found in the soil, can, on 
the other hand, be brought back to the hydrosol state by the addition of 
a small quantity of a suitable reagent, provided the residue has not been 
too thoroly dehydrated. 
Typical reversible colloids, after sufficient swelling has occurred, dis- 
solve in a solvent to give a homogeneous-appearing solution. Gum arabic, 
albumin, and ordinary glue are good examples. 
