CHAPTER XV 

 THE COLLOIDAL CONDITION 



REFERENCE has frequently been made, in preceding chapters, to 

 the fact that proteins, enzymes, lipoids, etc., exist in the protoplasm 

 of plants and animals in the colloidal condition. The properties 

 and uses of these compounds by plants depend so much upon this 

 fact that, before proceeding to the consideration of the actual 

 physical chemistry of protoplasm itself, it will be appropriate 

 and profitable to give some attention to the nature and sig- 

 nificance of the colloidal condition of matter and of some of the 

 phenomena which grow out of it. 



Every discussion of the colloidal condition in general properly 

 begins with reference to the work of the English physicist, Thomas 

 Graham, who carried on his investigations of the so-called " col- 

 loids " through a period of forty years, beginning with 1851. 

 His most important results were published, however, from 1861 to 

 1864. Graham studied the diffusibility of substances in solution 

 through the parchment membrane of a simple dialyzer. As a 

 result of his earlier investigations, he divided all the chemical 

 compounds which were known to him into two groups, which he 

 called " crystalloids " and " colloids," respectively, the first 

 including those substances which readily diffused through the 

 parchment membrane and the second those which diffused only 

 very slowly or not at all. He at first thought that crystalloids are 

 always inorganic compounds, while colloids are of organic origin. 

 He soon learned, however, that this distinction in behavior is not 

 always related to the organic or inorganic nature of the com- 

 pound. He further discovered that the same individual chemical 

 element or compound may exist sometimes in crystalloidal, and 

 sometimes in colloidal, form. This latter discovery led to the 

 conclusion that diffusibility depends upon the condition, rather 

 than upon the nature, of the material under observation. 



As a result of the long series of investigations which were 

 stimulated by Graham's work, the modern conception is that dif- 



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