MODE OF A CTION OF ENZ YATES. 3 1 7 
in glycerin, but it, has not been shown that the dried enzymes are 
soluble in anhydrous glycerin; on the contrary, Kiihne 1 states that 
pure trypsin is not soluble in strong glycerin ; and it is well known 
that, after precipitation l>y alcohol from glycerin extracts, the enzymes 
are afterwards much less soluble in glycerin. 2 
An elevated temperature rapidly destroys all enzymes when in 
solution, and it is of some importance that the temperature at which 
they are rapidly destroyed, although it varies considerably with the 
reaction of the solution, lies just a little below the range at which the 
bulk of the proteids coagulate. In the dried condition the enzymes are 
much more resistant to increased temperature, and can be heated to over 
100° C. for some time without losing their digestive properties on cooling 
and dissolving in water. 
The digestive action of the enzymes is not stopped by the presence 
of disinfectants, such as thymol, chloroform, or salicylic acid, in quantity 
sufficient to stop completely the action of organised ferments, particu- 
larly that of the putrefactive bacteria. 3 This fact has been turned to 
account practically in conducting prolonged digestion experiments, 
especially when the digestive action must be allowed to proceed in 
alkaline solution. 
Mode of action of enzymes. — The manner in which ferments bring- 
about the changes characteristic of them is very puzzling. The enzymes 
are altogether unaffected by the changes which they occasion, and, pro- 
vided the products of the action are not allowed to become concen- 
trated in solution, the ferment can work on indefinitely, and a finite 
amount of ferment can convert an infinite amount of material. The 
ferment may become by dilution, or unavoidable loss in manipulation, 
so weak that finally its action becomes inappreciable ; but before this 
happens it can be shown that it has converted a mass of material so 
many times greater than its own, that the idea that it undergoes any 
permanent alteration in the reaction which it induces must be abandoned. 
Thus, according to Hammarsten, 4 one part of rennin will curdle 400,000 
to 800,000 parts of milk ; while Petit 5 prepared a pepsin powder which 
in seven hours dissolved o00,000 times its weight of fibrin. 
There are numberless examples of chemical reactions, in which only 
well-known and much simpler compounds take a part, of a substance 
inducing a chemical reaction without itself becoming altered thereby. 
Such a substance is called a catalytic agent, and the reaction a catalysis 
or catalytic reaction. Ferment actions are such catalytic reactions, but 
when we say that ferments act catalytically the problem of how they 
act is not by any means solved ; we have merely found a name for it. 
In some cases, in which the presence of a substance is essential to a 
certain reaction, although this substance is not finally altered thereby, 
there is evidence that it is altered intermediately and rechanged again 
back to its initial condition during the reaction. 
Such a case is to be found in the action of sulphuric acid in the con- 
tinuous etherification process for producing ether from alcohol. It can 
be shown that the sulphuric acid first combines with part of the alcohol 
1 Verhaadl. d. naturh.-med. Vcr. zu Heidelberg, 1876, N. F., Bd. i. S. 196. 
2 v. Wittich, Arch./, d. ges. Physiol., Bonn, 1869, Bd. ii. S. 193. 
3 Kiihne, Verhandl. d. naturh.-med. Vcr. zu Heidelberg, 1876, N. F., Bd. i. S. 190. 
4 Jahresb. ii. d. Fortschr. d. Thier-Chem., Wiesbaden, 1877, Bd. vii. S. 166. 
5 Journ. de th&rwp., Paris, 1880. 
