culiar in not
undergoing hydrolysis except in very dilute solution and at a low
temperature. On cooling a hot soap solution, a jelly of more or less
firm consistence results, a property possessed by colloidal bodies, such
as starch and gelatine, in contradistinction to substances which under
the same conditions deposit crystals, due to diminished solubility of
the salt at a lower temperature.

Krafft (_Journ. Soc. Chem. Ind._, 1896, 206, 601; 1899, 691; and 1902,
1301) and his collaborators, Wiglow, Strutz and Funcke, have
investigated this property of soap solutions very fully, the researches
extending over several years. In the light of their more recent work,
the molecules, or definite aggregates of molecules, of solutions which
become gelatinous on cooling move much more slowly than the molecules in
the formation of a crystal, but there is a definite structure, although
arranged differently to that of a crystal. In the case of soda soaps the
colloidal character increases with the molecular weight of the fatty
acids.

Soda soaps are insoluble in concentrated caustic lyes, and, for the most
part, in strong solutions of sodium chloride, hence the addition of
caustic soda or brine to a solution of soda soap causes the soap to
separate out and rise to the surface. Addition of brine to a solution of
potash soap, on the other hand, merely results in double decomposition,
soda soap and potassium chloride being formed, thus:--

C_{17}H_{35}COOK + NaCl = C_{17}H_{35}COONa + KCl
potassium sodium sodium potassium
stearate chloride stearate chloride

The solubility of the different soaps in salt solution varies very
considerably. Whilst sodium stearate is insoluble in a 5 per cent.
solution of sodium chloride, sodium laurate requires a 17 per cent.
solution to precipitate it, and sodium caproate is not thrown out of
solution even by a saturated solution.

_Hydrolysis of Soap_.--The term "hydrolysis" is applied to any
resolution of a body into its constituents where the decomposition is
brought about by the action of water, hence when soap is treated with
_cold_ water, it is said to undergo hydrolysis, the reaction taking
place being represented in its simplest form by the equation:--

2NaC_{18}H_{35}O_{2} + H_{2}O = NaOH + HNa(C_{18}H_{35}O_{2})_{2}
sodium water caustic acid sodium
stearate soda