The substance may be combined with a variety of other chemical substances so that the action appears to be on several, but in reality it is on a definite group of molecules in each instance. For example, emulsin attacks several different glucosides but always sets free dextrose from them. 5. The action is inhibited and eventually stopped, and in some cases the enzyme is destroyed by an accumulation of the products of the action. If the products are removed, the action will continue, if the enzyme is not destroyed. This effect is explained partly because the enzyme probably combines with some of the products, since it does not act indefinitely, and partly because of the reversibility of the reaction. 6. Like many chemical reactions those of enzymes are reversible, that is, the substance broken up may be reformed by it from the products produced in many instances. Thus: maltose + maltase ⇆ glucose + glucose + maltase. fat + lipase ⇆ glycerin + fatty acid + lipase. 7. The presence of certain mineral salts seems to be essential for their action. These and other substances which are necessary are sometimes called _co-enzymes_. A salt of calcium is most favorable for a great many. 8. They may be adsorbed like other colloids by “shaking out” with finely divided suspensions like charcoal or kaolin, or by other colloids like aluminum hydroxide or proteins. 9. When properly introduced into the tissues or blood of an animal, they cause the body cells to form _anti-enzymes_ which will prevent the action of the enzyme (see Chapter XXVII). 10. Though inert, they show many of the characteristics of living organisms, that is (_a_) Each enzyme has an optimum, a maximum and a minimum temperature for its action. All chemical reactions have such temperature limits, the distinction is that for enzymes as for living substance the _range_ is relatively narrow. (_b_) High temperatures destroy enzymes. All in water are destroyed by boiling in time and most at temperatures considerably below the boiling-point. When dry, many will withstand a higher degree of heat than 100° before they are destroyed. (_c_) Temperatures below the minimum stop their action, though they are not destroyed by cold. (_d_) Many poisons and chemical disinfectants (Chapter XIV) which kill living organisms will also stop the action of enzymes, though generally more of the substance is required, so that it is possible to destroy the living cells by such means and yet the action of the enzyme will continue. (_e_) Most enzymes have an optimum reaction of medium either acid, alkaline or neutral, depending on the particular enzyme, though some few seem to act equally well within a considerable range on either side of the neutral point. _The final test for an enzyme is the chemical change it brings about in the specific substance acted on._