CHAPTER XIII 

 VISCOSITY 



The viscosity of protoplasm is intimately associated with 

 vital processes. Growth, reproduction, mitosis, amoeboid move- 

 ment, streaming, permeability, adsorption, metabolism, and 

 other cell functions all involve, and therefore are in part deter- 

 mined by, the viscous state of protoplasm. Change is as typical 

 of the viscosity of protoplasm as it is of life as a whole. Change 

 in protoplasmic consistency may range from the extreme fluidity 

 of a thin liquid to the firmness of a rigid jelly. The viscosity of 

 any one region in a cell may undergo pronounced changes with 

 change in physiological activities. Different regions in a cell 

 may also differ greatly in viscosity at any one time. To fail to 

 appreciate these facts is to fail to have a clear understanding of 

 protoplasmic behavior and the physiology of the cell. A viscosity 

 value of protoplasm is all but meaningless unless the species, the 

 kind of tissue, the physiological state, and the precise region in 

 the cell are specified. 



Dujardin, who with von Mohl first studied protoplasm 

 intensively, characterized it as a "living jelly," a "glutinous 

 substance"; and von Mohl described it as a "viscid mass." 

 Thus did both lay emphasis on the high consistency of the living 

 substance. 



Terminology. — Viscosity is that property of liquids which 

 causes them to resist flow. In the mind of the layman, it is more 

 characteristic of thick liquids such as molasses and pitch than of 

 thin ones such as water, though actually it is possessed by all 

 substances that flow, including gases. Viscosity is practically 

 sjmonymous with consistency and is the reciprocal of fluidity. 

 The importance of viscosity in the manufacture and use of paints, 

 lacquers, paper, rubber, cellulose, porcelain, and many other 

 types of plastic material has led to the naming of the study of 

 viscosity and thus establishing it as a distinct branch of physical 

 chemistry, viz., rheology — the science of flow. 



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