[5] Since the proteins are essentially _colloidal_ in nature, many of the terms used in the discussions of their properties, and these properties themselves, will be better understood after the chapter dealing with the colloidal condition of matter has been studied. A more logical arrangement so far as the systematic study of these properties is concerned would be to take up chapter XV before undertaking the study of the proteins (this order is actually followed in some texts on Physiological Chemistry). But from the standpoint of the consideration of the various groups of organic components of plants, it seems a better arrangement to consider these groups in sequence, and then to discuss the various physical-chemical phenomena which govern their activity. However, it is recommended that the student refer at once to Chapter XV for an explanation of any terms used here, which may not be familiar to him; and that after the study of Chapter XV, he return to this chapter dealing with the proteins for an illustrative study of the applications of the principles presented there. THE CLASSIFICATION OF THE PROTEINS Formerly, the classification of proteins was based almost wholly upon their solubility and coagulation reactions. More recently, since their products of hydrolysis have been extensively studied, their classification has been modified, in attempts to make it correspond as closely as possible to their chemical constitution and physical properties. As knowledge of these matters progresses, the schemes of classification change. On that account, no one definite scheme is universally used. For example, the English system varies considerably from the one commonly used by American biochemists, which is the one presented below. The proteins are divided into three main classes, as follows: (1) Simple proteins, which yield only amino-acids when hydrolyzed. (2) Conjugated proteins, compounds of proteins with some other non-protein group. (3) Derived proteins, decomposition products of simple proteins. The first two of these classes comprise all the natural proteins; while the third includes the artificial polypeptides and proteins which have been modified by reagents. These major classes are further subdivided into the following sub-classes, which depend in part upon the solubilities of the individual proteins, and in part upon the nature of their products of hydrolysis: 1. _The Simple Proteins_ A. Albumins--soluble in water and dilute salt solutions, coagulated by heat. B. Globulins--insoluble in water, soluble in dilute salt solutions, coagulated by heat. C. Glutelins--insoluble in water or dilute salt solutions, soluble in dilute acids or alkalies, coagulated by heat. D. Prolamins--insoluble in water, etc., soluble in 80 per cent alcohol. E. Histones--soluble in water, insoluble in ammonia, not coagulated by heat.