into a "gel," and redissolve upon heating into a colloidal solution or "sol"; and irreversible colloids, which, when heated with warm water, will coagulate at once into an unchangeable precipitate. Living protoplasm, as Darwin has shown in his experiments upon Drosera and other plants, acts exactly like a reversible colloid. Dead protoplasm, such as a coagulated blood clot, is an irreversible colloid consisting of a fixed network, the meshes of which contain the "sol." There is no evidence of internal structure in living protoplasm, and Hardy supposes that structure in dead protoplasm is produced by submortem or postmortem changes associated with coagulation. Whether the phase rule can be applied to colloids is still an open question bound up with the complex nature of bodies of which we know so little. But recently Siedentopf and Zsigismony have shown that colloidal metals, organic ferments and enzymes are systems in two phases of vast surface tension consisting of suspensions of ultra-microscopic particles acted upon by chemical, thermodynamic and electric potentials. Of such suspensions animal and vegetable bodies are largely made up, protoplasm being a sort of microscopic emulsion, the physiological action of which seems to be bound up with chemical, thermal, electric and osmotic changes between its semi-permeable membranes and surfaces of discontinuity and the various surface tensions and surface energies derived from the free energy of chemical or electric change. If we conceive of colloidal solutions as made up in this way, each tiniest particle being an ultramicroscopic furnace, retort or battery in itself and carrying a definite charge of electricity, we can understand how Liebig's theory of sympathetic vibrations might be applicable to colloidal catalysis at least, and how finely divided metals, serpent venoms or the excretions of microorganisms can produce the extraordinary effects they do. In close connection with the theory of catalysis is the nature of chemical purity and the fact that chemical changes rarely proceed directly to their final product, but usually pass through a series of intermediate stages. For a long time chemists have noticed that absolutely dry or pure substances will not interact directly upon each other, but the cooperation of a third substance is necessary for chemical change. Dried chlorine does not of itself act upon copper and other metals, but the presence of a little moisture will cause it to act upon them at once. A mixture of carbonic acid and oxygen is not explosive when thoroughly dry, but the slightest trace of steam will cause an explosion. The rapid solubility of zinc in sulphuric acid depends upon impurities in the former. Ebullition depends largely upon gaseous impurities in the boiling substance. Absolutely pure or distilled water has no digestive value, but, by its absorptive power, acts as an irritant or poison to the lining membrane of the stomach. Traces of moisture or other impurities have therefore a marked catalytic effect, a theory of catalysis which was first advanced as early as 1794 by Mrs. Fulhame in her "Essay
- Darwin, "The Power of Motion in Plants," passim.