The mechanical energy displayed in the movements of protoplasm is supplied by processes of chemical change, and chiefly of oxidation.

By these processes some of the substances contained in the protoplasm are destroyed, and their place must be supplied by fresh material. This material is obtained from the food, but, in order to render it available for the protoplasm, its atoms must be more or less disintegrated in order that they may again be assimilated. As Hermann very well puts it, the bricks of which the old house is built must be pulled asunder before they can be built up again into the new. In the present case, the bricks are the atoms of protoplasm in some other organism living or dead, which is being used as food by some larger mass of protoplasm, as, for example, a bacillus which has been absorbed by an amoeba. (Fig. 13.)

Fig. 13.   An amoeba figured at two different periods during movement. n, nucleus; i, ingested bacillus

Fig. 13. - An amoeba figured at two different periods during movement. n, nucleus; i, ingested bacillus.

In order to render the protoplasm in the bacillus available for the nutrition of the amoeba, the atoms of which it is composed must be, to some extent, decomposed. This process appears to be effected by enzymes or, as they are sometimes called, organic ferments.

Ferments are bodies which split up carbon compounds at moderate temperatures and lead to the formation of other carbon compounds, most of which are of a simpler constitution than the first.

In this definition we require to introduce the term ' moderate temperature,' because excessive heat alone will cause the atoms of a complex carbon compound to fly asunder and form simpler compounds, as in the process of dry distillation. A less heat than this, but aided by the action of powerful chemicals, will also produce the same effect. For example, fibrine heated with diluted hydrochloric acid under pressure yields peptones; but the same change is effected at the temperature of the mammalian body by the aid of pepsin. Trypsin from the pancreas effects a similar change when mixed with water alone without the aid of an acid, though its action is certainly aided by alkalies. Neither pepsin nor trypsin are alive, but they contain carbon, and are therefore called organic ferments. But this term easily leads to confusion with ordinary living or organised ferments, and so the term enzymes has been lately introduced to signify ferments such as diastase, ptyalin, and pepsin, which, though they contain carbon and are therefore called organic, are not alive and have no definite structure, or, in other words, are not organised. The term unformed ferments has also been applied to them.

By organised ferments we mean minute living organisms, hich in the course of their life-processes cause decomposition of the substances in which they live. They have also been called formed ferments. Examples of these are yeast and bacteria.

The processes of fermentation have been divided by Hoppe-Seyler into two kinds: (1) Those in which water is taken up; and (2) those in which oxygen is transferred from the hydrogen to the carbon atom.

The hydration in the first case is produced by the ferment acting either (a) like a dilute mineral acid at a high temperature, as in diastatic and invertive ferments and in the decomposition of glucosides; or (b) like caustic alkalies at a high temperature, as in the splitting up of fats or the decomposition of amide compounds. These processes of fermentation by hydration are chiefly carried on by enzymes.

The second class of fermentative changes by the transference of oxygen from the hydrogen to the carbon, as in lactic and alcoholic fermentation and in putrefactive processes, are chiefly produced through the agency of organised ferments. The action of the latter may be to a certain extent imitated by spongy platinum, which absorbs oxygen readily, and readily gives it off again to oxidisable substances. Thus acetic fermentation usually produced by an organised ferment may be also brought about by spongy platinum.

The products formed by the action of organised ferments on the media in which they live are poisonous to them; and when these products accumulate above a certain proportion, they kill the ferments. Just as a fire will be smothered in its own ashes, or an animal in a confined space will be poisoned by the carbonic acid which it has itself produced, so the yeast plant, when living in a solution of sugar, is killed by the alcohol which it produces, as soon as this amounts to 20 per cent; and other organised ferments have their lives limited in a similar way.