Page:Popular Science Monthly Volume 59.djvu/507

Rh

T has been said somewhat sententiously that the advance of science consists simply in a change of problems; we achieve progress when we substitute for one problem another at once more delicate and more precise. The recent history of the theory of alcoholic fermentation furnishes a conspicuous illustration of this aphorism. Liebig's ingenious conception concerning the breaking-down of the sugar molecule by the decomposing albuminous compounds in dead and dying yeast cells—his notion being that the sugar is toppled over, so to speak, by the mechanical shock of other falling molecules—was forced to yield to Pasteur's apparently clear demonstration of the part played under natural conditions by the living yeast cell. More recently the conception of the living cell as the essential feature of the process has been dethroned in its turn, and alcoholic fermentation is now shown to rest on the action of an 'unorganized,' 'lifeless,' or 'soluble' ferment or enzyme, secreted by the yeast plant. Further, the action of this enzyme and of other and more familiar 'unorganized' ferments has been brought into line with some of the most characteristic activities of the living cell, and many general life phenomena have been shown to be in reality phenomena of fermentation.

The consequent focusing of attention upon the enzymes, their nature, mode of action and chemical relations, has already been prolific in results of great biological interest. The science of experimental medicine, in particular, is discovering that many of the problems relating to immunity, to toxins, antitoxins and agglutinating substances are closely connected with the problems of fermentation and the enzymes.

So far as is known, the peculiar substances called enzymes are produced only by the living cells of animals and plants, although there are certain inorganic substances that so closely reproduce the essential qualities of enzyme action that they might almost be termed 'inorganic enzymes.' The enzymes obtained from the animal or plant cell have