112 L. V. HEILBRUNN 



changes in its physical state, so that any data obtained from extruded 

 protoplasm must be regarded with caution, and no certain conclusions 

 can be drawn from such data as to the viscosity of the protoplasm 

 before it was squeezed out of the cells that contained it. Some types 

 of plant protoplasm are in a continual state of flow within their cell 

 walls, and it might be thought that, from a study of the rate of such 

 movement, information could be gained as to the relative viscosity 

 of the protoplasm luider a variety of conditions. Unfortunately, 

 however, almost nothing is known as to the nature or magnitude of 

 the forces that govern such protoplasmic flow, so that a change in the 

 rate of flow following exposure to a given agent or condition might be 

 due either to a change in the propelling force or to a change in the vis- 

 cosity (or both). 



At the present time, there are only two standard types of method 

 commonly used for measuring protoplasmic viscosity. In one type 

 of method, granules or inclusions are moved through the protoplasm 

 by gravity or by centrifugal force. In the other type of method, the 

 speed of Brownian movement is used as a measure of viscosity. Heil- 

 bronn (12) has employed a magnetic method. He inserted small iron 

 wires into the pi'otoplasm of slime molds and then used an electro- 

 magnet to rotate them. This method is only applicable to slime mold 

 protoplasm (or perhaps also the protoplasm of the giant nerve fiber 

 of the squid). 



1. Gravity and Centrifuge Methods 



The two most widely used methods for studying the viscosity of 

 fluids are the capillary tube method and the falling sphere method. 

 As has been noted, the tube method is not appropriate for living 

 uninjured protoplasm. On the other hand, the falling sphere method 

 can be applied to many types of living cells. 



The falling sphere method depends on the application of Stokes' 

 law to the movement of a small sphere through a fluid. Derived 

 mathematically many years ago, Stokes' law has been widely used 

 in various fields of physics and physical chemistry. In its original 

 form Stokes' law is: 



W = Qir'qav 



in which W is the force pushing the sphere through the fluid, rj is the 

 viscosity of the fluid, a the radius of the sphere and v its velocity. If 



