44 THE POPULAR SCIENCE MONTHLY 



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 pro- 

 toplasm, as Darwin has shown in his experiments upon Drosera and 

 other plants, 103 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 post- 

 mortem 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 ultra- 

 microscopic furnace, retort or battery in itself and carrying a definite 

 charge of electricity, we can understand how Liebig's theory of sympa- 

 thetic vibrations might be applicable to colloidal catalysis at least, and 

 how finely divided metals, serpent venoms or the excretions of micro- 

 organisms 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 sub- 

 stances 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 solu- 

 bility of zinc in sulphuric acid depends upon impurities in the former. 

 Ebullition depends largely upon gaseous impurities in the boiling sub- 

 stance. 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 



103 Darwin, " The Power of Motion in Plants," passim. 



