COLLOIDS 31 



occurs to establish chemical ('quili])riuin. Chemical changes in the 

 crystalloids, by oxidation, nuhiclion or hych'olysis, upset this chem- 

 ical ecjuihbrium, and hence further diffusion, synthesis and hydrolysis 

 continue, one upsetting the other continuously. If equilibrium were 

 established we should have no further reactions, and the cells would 

 be inactive. The constant upsetting of the equUihrium is what con- 

 stitutes cell life. 



The relation of osmotic pressure and osmosis to physiological prob- 

 lems is only beginning to be studied. It is apparent that they must 

 be of essential importance in absorption from the ahmcntary canal, 

 in absorption and oxciction betAveen the cells and the blood stream, 

 and in secretion by glandular organs; but it is also certain that they 

 are no less important in all the less obvious chemical and physical 

 processes of the cell. 2° In pathological processes osmotic pressure 

 may play an equally important role, and the facts discussed in the 

 prececUng paragraphs will be alluded to frequently in subsequent 

 chapters. 



COLLOIDS-i 



Since Graham in 1861 studied the differences between the sub- 

 stances that chd or did not diffuse reachly through animal or parch- 

 ment membranes, soluble substances have been classified in the two 

 main groups of colloids and crystalloids, which distinction Graham 

 believed separated two entirely different classes of matter. Although 

 at the present time the differences between the two classes do not 

 seem so great, yet the same division is found useful in classification. 

 By colloids Graham indicated those substances which were dissolved 

 to the extent of showing no visible particles in suspension, but which 

 either did not pass through diffusion membranes at all, or did so very 

 slowly indeed, as compared to the crystalloid substances. Under cer- 

 tain conditions they tended to assume a sticky, glue-like nature, 

 hence the name. (Many substances are now known which have the 

 chief properties of the colloids and are therefore classified among 

 them, but never are glue-hke, e. g., the colloidal metals, so that the 

 name has lost some of its original significance.) The phj^sical prop- 

 erty which Graham particularly noted in the colloids, besides their 

 non-difTusibility was the tendency to assume various states of solidity. 



2° For further consideration of the subject of osmotic pressure in these rela- 

 tions, see: Livingston, "The Role of Diffusion and Osmotic Pressure in Plants," 

 University of Chicago Press, Chicago, 1903; Czapek, "Biochemie der Pflanzen," 

 Jena. Also, Spiro, Pauli and Hober, all previously cited. 



^' For full discussions of the nature of colloids see: Hober, " Physikalische 

 Chemie der Zelle," Leipzig, 1914; Pauli, Ergebnisse der Physiologie, 1907 (6), 

 105; Bechhold, "Colloids in Biology and Medicine," translated by J. G. M. 

 Bullowa, 1919; Wo. Ostwald, "Grundriss der Kolloidchemie," and "Theoretical 

 and Applied Colloid Chemistry," both translated by M. H. Fischer. A go9d 

 brief discussion of colloids is given by Young in Zinsser's "Infection and Resis- 

 tance." 



