APPENDIX 



COLLOIDS 



Crystalloids and Colloids. — In the older literature, sul)stances 

 were divided into crystalloids and colloids. At the present time, 

 it is thought that the term ''colloid" should be applied to the state 

 of the substance and that so-called crystalloids may be prepared in 

 the colloidal state. We now speak of substances being in true solu- 

 tion and in the colloidal state. 



True Solution. — In the case of true solutions, one is dealing 

 with a solvent such as water and a dissolved substance, as e.g., 

 sodium chloride, glucose, etc. When sodium chloride goes into 

 solution in water, it tends to ionize, i.e., the molecule breaks up 

 into sodium and chloride ions, while glucose goes into solution 

 in the molecular state. Because of this difference, the sodium 

 chloride tends to diffuse through a membrane more rapidly than 

 glucose, although both are diffusible. It is estimated that the size 

 of a molecule of sodium chloride is 5.6 x 10"^ cm. while that of 

 glucose is 14.7 x 10~* cm. It is obvious that the glucose molecule 

 is considerably larger than that of sodium chloride and that it is 

 also of a different quality. 



Colloidal State. — If sodium chloride or glucose could be present 

 in the water not in the io7iic or molecular state but in particles 

 or aggregates ranging in size from 0.00001 em. to 0.0000001 cm., it 

 would be present in the colloidal state. The water would be called 

 the "dispersion medium" and the small particulate substance 

 dispersed in it would be called the "dispersed phase." 



Homogeneous and Heterogeneous Systems. — In nature we find 

 that such su])stances as the serum globulins and albumins and the 

 cell protoplasm are found in the colloidal state. True solutions are 

 also spoken of as "homogeneous systems" in which there are no 

 apparent physical surfaces of discontinuity, while colloids are 

 "heterogeneous systems" consisting of two or more phases sepa- 

 rated from each other by surfaces of discontinuity. Alexander 

 cites Zsigmondy's observation that perhaps this basis of differentia- 

 tion between homogeneous and heterogeneous systems is more 



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