ing of at least two reactions, in the first of which the ferment forms with the substance to be decomposed a compound which is split up in the second with the regeneration of the ferment. This has not been experimentally proved for the real ferments, but it explains these processes so simply and completely that its correctness can not be doubted.
These processes of fermentation have an extraordinary distribution, for not only do the individual living cells produce ferments, but also those which form parts of complicated organisms. This is especially the case with those cells which are often united in enormous quantities to form a larger organ. I will recall the salivary gland, the peptic gland, and the pancreas, whose secretions are exceedingly rich in ferments. This is of the greatest importance for the economy of the animal organism, since it permits a considerable performance with a comparatively small expenditure of means.
I wish to call attention to another important peculiarity of the processes of fermentation—that is, to their sensibility to foreign influences. Some will only take place in a completely neutral or slightly alkaline solution, others only in a slightly acid solution; they all progress most rapidly at a certain temperature, and even slight deviations from the most favorable conditions are sufficient to sensibly retard the action of a ferment or to completely arrest it. The pepsin of the gastric juice acts exclusively in acid solution (best in hydrochloric acid); hence, if the gastric juice contains no free acid or only an organic acid, such as lactic acid, the pepsin will produce no effect on albumen, or only in a slight degree.
Although I have just described this sensibility as a peculiarity of the process of fermentation, it is not to be inferred that it is noticed here exclusively; on the contrary, we find it in all chemical processes, only in a less degree. How many reactions take place only at a given temperature, or at a given degree of concentration? Creatine, for instance, when boiled with baryta-water, is partly decomposed into urea and sarcosine, and partly into ammonia and methylhydantoin; and it is evident that the possibility of splitting up in different directions is increased with the size of the molecule. In the organism a large number of bodies are present which have a very complicated structure, and are therefore readily decomposed. They have an ephemeral existence only, but are, nevertheless, of the greatest importance in the economy of the whole. Most of them are unknown to us, but in some cases they have been successfully isolated. I will only mention glycogen, the discovery of which is several decades old, and the purple of the retina, which has only lately been recognized. All such complicated compounds, having a high molecular weight, are capable of furnishing very different products of decomposition under slightly modified conditions. If, therefore, the decompositions in the organism should always take place in the same manner, the governing conditions must be exceedingly constant. Every product of decomposition, which is
