Since enzymes are catalysts, it is plain that an accurate description of their activity should, in each case, refer to the influence which they exert upon some definite reaction velocity. But since the phrases necessary to describe such an effect are cumbersome and inconvenient, and since most of the reactions which are accelerated by the catalytic action of enzymes are either simple hydrolyses, changes in oxygen content, or other simple decompositions or condensations, which will otherwise proceed so slowly as to be practically negligible, it is customary to speak of the enzyme as "acting upon" the material in question. It should be understood, however, that this is a misstatement, as the enzyme cannot actually initiate a reaction, or "act upon" any substance; it only acts as a catalyzer to accelerate the action of water, oxygen, etc., upon the material in question. Generally speaking, most enzymes are colloidal in form and, hence, do not diffuse through membranes such as the cell-walls. Some of them perform their characteristic functions only within the cell, or organ, which elaborates them, and can be obtained outside these tissues for purposes of study only by first rupturing the cell-wall or other membrane with which they are surrounded. Such enzymes are known as "intracellular." Others are regularly secreted by glands which discharge them onto other organs, as the stomach or intestines of animals, where they perform their useful functions; or, as in the case of germinating seeds, they move to other parts of the organ, and can be extracted from the tissue by simple treatment with water. These are known as the "extracellular" enzymes. Enzymes are specific in their action. Any given enzyme affects only a single reaction; or at most acts only upon a single group of compounds which have similar molecular configuration. Usually it is only a single compound whose decomposition is accelerated by the action of a particular enzyme; but there are a few enzymes, such as _maltase_ (which acts on all [alpha]-glucosides) and emulsin (which acts on all [beta]-glucosides) which act catalytically upon groups of considerable numbers of similar compounds. Enzymes, like all other catalysts, act more energetically at increased temperatures; but for each particular enzyme there is an "optimum temperature," (usually between 40° and 65°) above which the destructive effect of the temperature upon the enzyme itself more than offsets the accelerating influence of the increased temperature. At still higher temperatures (usually 80° to 100°) the enzymes are "killed," i.e., rendered permanently inactive. All enzymes are "killed" by boiling the solutions in which they are contained. Dry preparations of enzyme material can withstand somewhat higher temperatures, for somewhat longer periods of time, than can the same enzyme in moist condition or in solution. When an enzyme has once been inactivated by heating, or "killed," it can never be restored to activity again.