Page:Popular Science Monthly Volume 67.djvu/274

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T was by the well-known investigations of the English physicist, Graham, published in the seventh decade of the last century, that the general attention of scientists was first drawn to the existence of a class of homogeneous mixtures, differing materially in their properties from ordinary solutions, such as those of salt and sugar. Impressed by the fact that the dissolved substance as a rule separates from the one class of solutions in the amorphous and often gelatinous state, and from the other in the form of crystals, he designated the former substances colloids and the latter crystalloids, and their solutions have since been commonly known, respectively, as colloidal and as crystalloidal or ordinary solutions. During the period immediately following Graham's classical researches, the subject of colloidal solutions received comparatively little attention. Within the last fifteen years, however, this field has become a favorite hunting ground of both physical chemists and physiologists in their searches after new truths, and greatly has the store of our knowledge in regard to this important state of aggregation been thereby increased. Yet the difficulty in reaching general conclusions as to the properties of these solutions has proved to be a very great one, owing to the complexity of the phenomena and to the apparent contradictions between many of the results obtained with different colloids and by different investigators. Moreover, the original literature of the subject has become so extensive and so detailed as to be almost overwhelming to one who, with limited time to devote to it, desires to obtain a general survey of this field of work. A brief review of some of the more important principles thus far established may, therefore, be of general interest.

It seems appropriate to begin the consideration of the subject with a definition of the class of substances to which our attention is to be devoted. In accordance with the general use of the term, colloidal mixtures are most simply defined as liquid (or solid) mixtures of two (or more) substances which are not separated from one another by the