Much, if not all, of their physiological activity is due to their colloidal nature, the importance and effects of which will be more apparent after the chapters dealing with the colloidal condition of matter and with the physical chemistry of protoplasm have been studied. REFERENCES ABDERHALDEN, E.--"Neuere Ergebnisse auf dem Gebiete der Speziellen Eiweisschemie," 128 pages, Jena, 1909. FISCHER, E.--"Untersuchungen über Aminosäuren, Polypeptide, und Proteine, 1899-1906," 770 pages, Berlin, 1906. MANN, G.--"Chemistry of the Proteids," 606 pages, London, 1906. OSBORNE, T. B.--"The Vegetable Proteins," 138 pages, _Monographs_ on Biochemistry, London, 1909. PLIMMER, R. H. A.--"The Chemical Constitution of the Proteins, Part I, Analysis," 188 pages; and "Part II, Synthesis, etc." 107 pages, _Monographs_ on Biochemistry, London, 1917. (3d ed.). ROBERTSON, T. B.--"The Physical Chemistry of the Proteins," 477 pages, New York, 1918. SCHRYBER, S. B.--"The General Characters of the Proteins," 86 pages, _Monographs_ on Biochemistry, London, 1909. UNDERHILL, F. P.--"The Physiology of the Amino-acids," 169 pages, 13 figs. 1 plate. Yale University Press, 1915. CHAPTER XIV ENZYMES AND THEIR ACTION The characteristic difference between the reactions of inorganic compounds and those of organic substances lies in the rapidity, or velocity, of the chemical changes involved. Speaking generally chemical reactions take place between substances which are in solution, so that they may come into sufficiently intimate contact that chemical action between them can take place. There are, of course, occasional examples of reactions between dry solids, such as the explosion of gunpowder, etc., but the general rule is that reacting materials must be in either colloidal or true solutions. Inorganic materials, when dissolved in water, usually ionize very readily. That is, they are not only disintegrated into individual _molecules_, but a considerable proportion of these molecules separate into their constituent _ions_. When solutions containing ionized compounds are brought together, conditions for chemical interaction are ideal, and the reaction proceeds with such tremendous rapidity as to be completed almost instantaneously, in most cases.