CHAPTER XXI 



COLLOIDS 



BY STEWART W. YOUNG 

 Professor of Physical Chemistry, Stanford University, Cal. 



INTKODUCTOEY 



IN attempting to give in the brief space of a single chapter any 

 adequate account of the present state of our knowledge in so vast a 

 field as that of colloid chemistry and physics one is confronted with 

 a rather difficult problem. In the present outline the attempt will 

 be made to get at some notion of the matter by a presentation first 

 of the more important generalizations which have been drawn, this 

 to be followed in each case by sufficient experimental evidence to 

 serve as illustration, together in some cases perhaps with certain 

 evidence which may seem to contradict in some degree such cur- 

 rent conceptions. The reason for this particular method of pres- 

 entation lies in the fact that new material is so rapidly accumu- 

 lating, much of which seems more or less at variance with 

 present accepted theories that it seems more than possible that 

 some of these fundamental generalizations may soon undergo ma- 

 terial modification if not reform. It would, therefore, seem ill- 

 advised, in presenting a brief resume to readers who are not 

 physicists or chemists, merely to present the present theories as 

 they are used to-day by workers in the field, and to sound a note 

 of warning that many, if not all of them, are not so securely 

 supported by broad evidence as to allow of very concrete prediction 

 being based upon them. 



Definition. The fundamental distinction between the crystalloid 

 and colloid states of matter was first drawn by Thomas Graham * as a 

 result of his investigations into the phenomenon of dialysis. He 

 noted that in general those substances which when in solution did not 

 pass through the dialyzing membrane, or did so only very slowly, also 

 were characterized by the fact that when they separated from solu- 

 tion, either by precipitation or by evaporation, they did so in the non- 



1 Graham. Phil Trans., 1861, 183. 



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