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drive off the combined water, it has been thought by some to be due to water of combination. Others have attributed plasticity to the action of bacteria, but Hecht and Kosmann failed to increase the plasticity of clays by inoculation. Stover, however, claims to have done this in 1903 by treating a thin clay with water drained from a fat clay, and he attributes the action to the specific influence of bacillus sulphureus. It is difficult to under- stand how a bacillus whose function is to decompose pyrites should promote plasticity except indirectly by removing salts which are detrimental to the phenomenon. In any case the smell arising from the process is likely to prevent its general acceptance. Bischof opines that felting of the particles of clay enables them to adhere to each other. Numerous other theories have been put forth from time to time, and they are comprehensively surveyed by Zschokke (Bau materialen Kunde, 24, 7). At present three theories contest the field the colloidal, the soluble salt, and the molecular attraction theories. Taking the colloid theory first, a colloid may be defined as anything not crystalline. Colloids exist in two charac- teristic forms, sol and gel. The homogeneous suspension of a colloid in a liquid is called sol ; the continuous jelly with pore walls and pores filled with a liquid is called gel. Colloids are reversible or irreversible, according to whether or not they will pass back from gel to sol. The process of passing back from sol to gel is called coagulation, and