Viii PREFACE 



tical isomerism which is displayed by the simpler carbohydrate 

 radicals out of which they are built up, while, on the other hand, 

 the behavior of rubber and its congeners is primarily conditioned 

 by the enormous internal molecular friction which leads them to 

 display, to an exaggerated extent, phenomena analogous to hys- 

 teresis which, although not wholly negligible in other colloids, for 

 example in the protein group, nevertheless seldom present them- 

 selves as salient characteristics of their behavior. 



The colloids are therefore an exceedingly heterogeneous group, 

 the only common distinguishing characteristic being the relatively 

 enormous mass and volume of the molecules of the most "typical" 

 representatives of the class, and of course as many gradations of 

 behavior exist as there are gradations in the mass and volume of 

 molecules. Nothing is to be gained, therefore, by endeavoring to 

 force the various members of the colloid group into artificial con- 

 formity with definitions which are designed to separate them, as 

 if they were a homogeneous group, from other classes of chemical 

 substances. To describe a particular property or mode of be- 

 havior as a "colloidal phenomenon" neither defines nor inter- 

 prets it and furthermore fails even to describe it, since there are 

 no phenomena which are distinctively "colloidal" and displayed 

 by every member of the colloid group, saving only those phenomena 

 which depend primarily upon the simple factor of the mass or 

 volume of the molecules, and which are therefore predictable 

 from and implied in the properties or behavior of the smaller 

 molecules of the non-colloidal substances. 



Similarly, the use of the term "adsorption" to describe the 

 union between colloids and other substances impfies a uniformity 

 where no uniformity exists and is, moreover, devoid of utility or 

 meaning unless we attach to the definition some distinct idea of 

 the nature of the underlying forces which condition the union, 

 whether these forces be regarded as consisting of chemical {i.e., 

 atomic) attractions or of capillary {i.e., molecular) attractions. 

 But in forming such concrete ideas we are simply returning to 

 conceptions which are already familiar to us in the "crystalloid" 

 field of chemistry and which call for no definitions which we do not 

 already possess as the result of our general acquaintance with the 

 physical and chemical phenomena which are displayed by simpler 

 and hitherto more familiar substances. 



The investigations of recent years, not only upon the behavior 



