In natural processes the best known type of an enzyme is diastase (amylase), the enzyme contained in malt, and which enables the malt to convert starch into dextrin and sugar (maltose). It is capable of transforming more than 2000 times its own weight into sugar, which fact is quite sufficient to show that its action differs from that of an ordinary chemical reaction. Another enzyme, sucrase, according to O’Sullivan and Thompson, will hydrolyze 100,000 times its weight of cane sugar to invert sugar. Rennet will coagulate 250,000 times its own weight of casein in milk. The list of enzymes grows longer almost daily, as some new one is separated having a specific action, until one is almost led to believe that the mechanism of life itself, as manifested in the cell, is due to enzymes. It has been found that enzymes act very much in the same way as inorganic catalysers. As an example, the velocity of the reaction of invertase (the enzyme of yeast which hydrolyses cane sugar to grape sugar) has been compared with the same hydrolysis brought about by heating a solution of cane sugar with a mineral acid. In both cases the reaction is in accordance with the law of mass action (Guldberg and Waage) that the amount of sugar transformed will decrease as less remains to be transformed. In the diagram (Fig. 26) the curve A is for invertase (Jas. O’Sullivan, “Journ. Inst. of Brewing,” vol. v. p. 168); curve B is for the hydrolysis by acid (Wilhelmy), from which it will be seen that the manner in which the hydrolysis proceeds is practically the same in both cases.[96] [96] There are apparent exceptions and complications of this law which we shall not here enter into, except to say that they may be explained by the fact that the action of some enzymes is reversible (see p. 141, under lipase.) [Illustration: Fig. 26.--Curves showing Rate of Hydrolysis.] In the case of fermentation by the living organism, the fermentation rises rapidly, and then gradually slows down and comes to an end before the whole of the fermentable matter is used up; the curve, therefore, is of a hyperbolic character, C in the diagram, which represents, in a general way, the fermentation of glucose by B. furfuris. The ordinates represent the amount of acid produced by the bacteria. The time in this case would be more nearly represented by hours instead of minutes on the abscissa. The mathematical expression for the velocity of the reaction is _dx_/_dt_ = _k_(_a_ − _x_) where _a_ = original concentration of solution _x_ = the quantity transformed in time t _k_ = coefficient of velocity of the reaction By integrating the above equation, it may be shown that 1/_t_ log(_a_/(_a_ − _x_)) = _k_ For the experiment of J. O’Sullivan with invertase, _k_ has a mean value of 0·0013; for Wilhelmy’s experiment with acid, _k_ has a mean value of 0·001377.