It must not be supposed that to Pasteur is due the whole credit of the knowledge acquired respecting the cause of fermentation. He did not first discover these living organisms; he did not first study them and describe them; he was not even the first to suggest that they were the cause of the processes of fermentation or disease. But, nevertheless, it was Pasteur who "first placed the subject upon a firm foundation by proving with rigid experiment some of the suggestions made by others." Thus it has ever been in the times of new learning and discovery: many contributors have added their quota to the mass of knowledge, even though one man appearing at the right moment has drawn the conclusions and proved the theory to be fact. In order that no confusion may arise in the mind of the reader, we may here say that, although fermentation is always due to a living agent, as proved by Pasteur, the process is conveniently divided into two kinds.[30] (1) When the action is direct, and the chemical changes involved in the process occur only in the presence of the cell, the latter is spoken of as an _organised ferment_; (2) when the action is indirect, and the changes are the result of the presence of a soluble material secreted by the cell, acting apart from the cell, this soluble substance is termed an _unorganised soluble ferment_, or _enzyme_. The organised ferments are bacteria or vegetable cells allied to the bacteria; the unorganised ferments, or enzymes, are ferments found in the secretions of specialised cells of the higher plants and animals. With the former this book deals in an elementary fashion; with the latter we have little concern. It will be sufficient to illustrate the enzymes by a few of the more familiar examples. They form, for example, the digestive agents in human assimilation. This function is performed, in some cases, by the enzyme combining with the substance on which it is acting and then by decomposition yielding the new "digested" substance and regenerating the enzyme; in other cases, the enzyme, by its molecular movement, sets up molecular movement in the substance it is digesting, and thus changes its condition. These digestive enzymes are as follows: in the saliva, _ptyalin_, which changes starch into sugar; in the gastric juice of the stomach, _pepsin_, which digests the proteids of the food and changes them into absorptive peptones; the pancreatic ferments, _amylopsin_, _trypsin_, and _steapsin_, capable of attacking all three classes of food stuffs; and the intestinal ferments, which have not yet been separated in purer condition. In addition to these, there are ferments in bitter almonds, mustard, etc. Concerning these unorganised ferments we have nothing further to say. Perhaps the commonest of them all is _diastase_, which occurs in malt, and to which some reference will be made later.