never been isolated in a pure condition, and hence no definite knowledge of their chemical composition and constitution has yet been secured. In general, enzymes are precipitated by alcohol from the watery extract of plant or animal tissue, but such a precipitate contains the enzyme inextricably mingled with other chemical compounds, and all attempts to separate out a pure enzyme have thus far signally failed. The whole field of enzyme study is therefore at present beset with the same sort of difficulties that the science of bacteriology encountered before the days of 'pure cultures,' and it is doubtless true that effects that are at the present time ascribed to individual enzymes are in reality caused by mixtures of distinct varieties.
The different kinds of enzymes are at present chiefly distinguished through the differences in the changes that they produce in other substances. The enzymes that act upon starchy substances, for instance, may be conveniently grouped together; those that disintegrate albuminous compounds may be similarly treated, and so on.
Enzymes converting starchy substances into sugar. The longest known and perhaps most thoroughly studied enzyme is a representative of the group that transforms insoluble carbohydrate substances into soluble ones. Amylase, or diastase, is the well-known enzyme that accomplishes the conversion of the starch of the barley-grain into sugar in the process of malting. The action of the enzyme in this process appears to be quite elaborate since the complex starch molecule passes through several stages during its conversion into sugar, the hardly less complex substances known as 'erythrodextrins' and 'achroödextrins' being formed on the way. The theory has been advanced that the starch molecule breaks down by the taking on of successive molecules of water and by subsequent decompositions, sugar (maltose) being formed at each splitting, together with a dextrin of lower molecular weight. Duclaux, however, maintains the existence of two enzymes in malt, one, a liquefying or decoagulating enzyme to which he would restrict the name amylase, and which converts the insoluble starch into the soluble dextrins, and a second (dextrinase), which has a saccharifying power and converts the dextrins into sugar. Other enzymes that may be placed in the same group with amylase are inulase and cytase. Inulase converts into fruit sugar a reserve food-substance found in many plants and known as inulin. Inulin is allied to starch in its chemical composition, and probably breaks down by successive stages under the action of inulase just as starch does under the influence of amylase. Another form of reserve food-substance stored up by many plants is the familiar substance comprising the cell-wall of most plants and known as cellulose. This substance, like inulin and starch, can be changed into a more directly utilizable substance by the action of cytase, an enzyme found
