116 L. V. HEILBRUNN 



the influence of gravity. But sometimes, when injury is at a mini- 

 mum, the injected oil drops rise through the protoplasm, and in these 

 instances, by measuring the speed of movement, viscosity determina- 

 tions can be made. 



Centrifuge Method. This method has been much more widely 

 used than the gravity method. The principle underlying the 

 method is the same as for the gravity method, except that the move- 

 ment of particles through protoplasm is under the influence of cen- 

 trifugal force rather than gravity. In using Stokes" law, therefore, 

 we must substitute for g the term eg, in which c is the centrifugal 

 force in terms of gravity. The formula then becomes : 



2cg{(r — p)a^ 



i> — 



9i7 



For a given centrifuge, c can be calculated if we know the number of 

 turns per second (n) and the radius of the circle described by the ends 

 of the centrifuge tubes (r). Then: 



c = A7rhih-/g 



One advantage of the centrifuge method is that it makes possible 

 measurements of viscosity at different rates of shear, for it is only 

 necessary to vary the centrifugal force in order to obtain a wide range 

 of forces. This has both a theoretical and a practical advantage. 

 Often, it may be desirable to know whether or not the protoplasmic 

 viscosity varies with rate of shear. Practically, one chooses a cen- 

 trifuge that gives a convenient speed of movement of the protoplasmic 

 granules. 



Generally, the best type of centrifuge to use is a hand centrifuge. 

 Electric centrifuges do not start or stop promptly, and with them it is 

 scarcely possible to apply a known amount of centrifugal force for 

 short time intervals. Years ago, various types of hand centrifuges 

 were on the market. Now, it is difficult to obtain hand centrifuges 

 that turn rapidly enough. Such centrifuges may be obtained from 

 the J. H. Emerson Co., Cambridge, Mass. Any type of hand cen- 

 trifuge used should be equipped with some sort of a protective device 

 in case of tube breakage. The head of the centrifuge should be ar- 

 ranged so as to hold small glass tubes. Ordinarily tubes of about 4-5 

 mm. outside diameter are used. It is important to have the tube 

 free from glass dust. The ends of the tubes are sealed in a flame some 

 time before they are ready for use. If soft glass is used, this can be 



