The differential quotient dQ/dt gives the velocity for each moment, and it will be seen that the swelling becomes slower and slower as the maximum is approached. Thus, the law of the velocity of swelling is identical with that of the velocity of inversion of cane sugar, itself an application of the law of masses.[56]
For every process of swelling, the constant K must be determined experimentally from observation of M, and it may be shown that—
K = 1/t log M/M – Q
whence the value of K may be calculated for each series of determinations.
On the manner in which water is held in swollen colloid bodies, three hypotheses have been put forward.[57]
1. The hypothesis that colloids have a structure in the form of a honeycomb. (Bütschli.)
2. The water is absorbed at the surface of colloids in a specially condensed form.[58]
The water forms, with the swollen body, a “solid solution.” (Nägeli.)
Swelling, and its opposite, contraction, are connected with the surface tension between the swelling or contracting bodies and the surrounding solution. With diminished surface tension, the surfaces of contact between the two become greater, i.e. swelling takes place, with simultaneous diminution of the volume of the whole system, and vice versa.
For the absorption phenomena which occur, the following relation holds good: When a substance in solution diminishes the surface tension at the dividing surface, its concentration is increased; it is absorbed. When a substance in solution increases the surface tension at the dividing surface, its concentration is diminished.