268 POPULAR SCIENCE MONTHLY. 



THE PKEPAKATION AND PROPEKTIES OF COLLOIDAL 



MIXTUEES.* 



By Professor ARTHUR A. NOYES, 



MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



IT 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 sub- 

 stances colloids and the latter crystalloids, and their solutions have 

 since been commonly known, respectively, as colloidal and as crystal- 

 loidal or ordinary solutions. During the period immediately follow- 

 ing 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 phys- 

 ical 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 de- 

 tailed 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 estab- 

 lished 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 



* This article is based upon a presidential address delivered by the author 

 at the Philadelphia Meeting of the American Chemical Society, December 29, 

 1904. 



