the cells without injuring the enzymes, then the
material is minced or ground up and suspended in water containing the
antiseptic, until the enzymes dissolve the cell-walls and so escape into
the liquid--this process being especially adapted to the preparation of
active extracts from yeasts, which contain the necessary cell-wall
dissolving enzymes to facilitate autolysis.

Enzymes may be separated out of the aqueous extract obtained from cells
ruptured by any of the above methods, by precipitation with alcohol,
acetone, or ether, in which they are insoluble; but if this is done, the
precipitate must be at once filtered off and rapidly washed and dried, as
prolonged contact with these precipitating agents greatly diminishes the
activity of most enzymes. Or, they may be adsorbed out of solution on
gelatinous, or colloidal, materials, like aluminium hydroxide, or various
hydrated clays. If the dry preparations obtained in any of these ways are
contaminated by carbohydrates, proteins, etc., these may be removed by
treatment with suitable digesting enzymes obtained from the saliva,
gastric, and pancreatic juices, and the digested impurities washed out with
60 to 80 per cent alcohol, leaving the enzyme preparation in a purified but
still active form.

In any study of the "strength," or possible catalytic effects, of an enzyme
preparation, it is necessary, first, to determine what particular reaction
it affects, by qualitative tests with various substrate materials, such as
starch, sugars, glucosides, proteins, etc., and then to determine
quantitatively its accelerating effect upon the reaction in question. The
latter may be done by measuring either the _time_ required to carry a unit
quantity of the substrate material through any determined stage of chemical
change, or the _quantity_ of the substrate which is changed in a unit
period of time. It would not be profitable to go into a detailed discussion
here of the methods of making these quantitative measurements of enzyme
activity. Such discussions must necessarily be left to special treatises on
methods of study of enzyme action. It may be said, however, that generally
both the qualitative tests for, and the quantitative measurements of, the
accelerating influence of enzymes depend upon the observation of some
change in the physical properties of the substrate material, such as the
optical activity, electrical conductivity, or viscosity, of its solution.
In some cases, it is