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 VITALISM : A BRIEF HISTORICAL AND CRITICAL REVIEW. 325 owing to the fact that they occur in inappreciable quantity, are recognisable only by their effect. Like protoplasm, they have never been synthetically prepared, nor have they been isolated in a pure state. To bodies of this class have been given the name of ferments or enzymes. 1 The recent researches of Euiil Fischer (24) and others on ferment-action have brought to light many curious details of the process. It has been found, for instance, that one ferment will com- pletely convert a given glucoside into sugar, while another ferment will only convert it in part ; and on the other hand, that one of two bodies having the same molecular composi- tion but slightly different molecular configurations will be zymolised by a given ferment, which will leave the other body untouched. 2 Fischer was led to consider this appar- ently selective ability on the part of enzymes during his laborious investigations into the synthesis of the sugars. Availing himself of the past work of Le Bel and van't Hoff on the relation between optical activity and the asymmetry of intramolecular carbon-atoms, Fischer sought to establish the direct dependence of (hydrolytic) ferment-action upon molecular configuration. He showed how it was possible for optically active bodies to combine with other simpler sub- stances and to build up still more complex optically active bodies. He demonstrated the profound influence on zymo- lysis exerted by racernism or other optical modifications, and finally he concluded that the same definite relation must subsist between the molecule of glucoside and the molecule of ferment, which obtains between a lock and its key. The door of chemical reaction can only be opened when the two molecules fit thus accurately together. At that moment molecules of still greater size are formed, until finally, aided by the reaction of the external medium and the simultaneous addition of the elements of water, they break down into the original enzyme-molecules g,nd the products of the ferment- action. A similar train of thought appears likely to throw con- siderable light on the mode of growth of living substance. 3 1 Of the two kinds of ferment, organised and unorganised, only the latter is dealt with throughout this article. It is fast becoming recognised that the action of so-called organised ferments has a twofold character, resulting partly from direct metabolic (excretory) changes within the organism and partly from the influence of secreted unorganised ferments. 2 Zymolysis is the action of a ferment or enzyme. Glucosides are substances* yielding sugars by the action of alkalies, acids or enzymes.
 * In this connexion I gladly acknowledge my obligations to Dr. Ruhe-

mann of Cambridge. Since the above was written, further chemical speculations and discussions have appeared, which necessarily find no mention here.