IV. VISCOSITY MEASUREMENTS 121 



the thickness of this viscous coiiox increases at the exi)eiisc of the 

 protoplasm in the interior. Some agents, indeed, cause a progressive 

 thickening of the cortex, until most of the protoplasm becomes cor- 

 tex. 



Although egg cells and some protozoa are suitable for centrifuge 

 studies, many other types of cells are not. Attempts have been made 

 to study the viscosity of protoplasm of frog muscle cells by the centri- 

 fuge method — without success. Epithelial cells are small and thus 

 difficult to study. There is no published work on the centrifugal 

 force needed to move granules through various types of nerve cells. 

 Many plant cells might provide suitable material. The chloroplasts 

 can be moved through the protoplasm of the cells of the leaves of 

 the water plant Elodca. In some algae, various structures can be 

 seen to move through the cell under the influence of centrifugal force. 

 Thus, Northen (21) observed the movement of chromatophores in 

 Spirogyra. Presumably, when these structures are moved through 

 the cell, their movement is due to a tearing loose from attachments. 

 Stokes' law" can scarcel^^ be used for such a movement and, although 

 the information obtained by Northen is of interest in the interpreta- 

 tion of physical changes in protoplasm, it can hardly give direct or 

 certain information as to viscosity change in the main mass of the 

 protoplasm. 



2. Brownian Movement Method 



Brownian movement of living material has been known for a long 

 time. As a matter of fact Brown's discovery of the motion was really 

 a discovery of the fact that movement could occur in nonliving as well 

 as living material. Soon after the middle of the nineteenth century, 

 students of protoplasm made observations on the Brownian move- 

 ment of particles within the cytoplasm and the nucleus, and some of 

 these workers recognized clearly that the rate of this movement was an 

 index of the fluidity of the protoplasm. But, in general, the earlier 

 observations were not very exact. Various conditions were seen to 

 affect the rapidity of the movement; and indeed sometimes the 

 movement seemed to cease entirely. Such cessation of movement 

 could be taken to indicate a marked increase in the viscosity of the 

 protoplasm. 



Fortunately, by the application of Einstein's equation for Brown- 

 ian movement, it has been possible to obtain more exact estimates of 



