IV. VISCOSITY MEASUREMENTS 117 



done very rapidly. A large supply of tubes is needed and care should 

 be taken so that the tubes of any given pair are of approximately 

 equal weight. 



Some years ago, Harvey (20) described a microscope centrifuge, 

 and, for a time, this apparatus was manufactured commercially. 

 With Harvey's centrifuge, it is to some extent possible to \yatch 

 granules moving through a cell under the influence of centrifugal force. 

 Unfortunately, however, the cells being observed rarely stay in the 

 field, so that only occasionally is one able to make measurements of 

 the speed of movement of the granules. 



With an ordinary centrifuge, it is necessary to make series of tests 

 in order to determine the speed of the moving granules. Thus, for 

 example, in a recent study of the viscosity of the protoplasm of the 

 egg of the worm Chaetopterus {20a) the centrifuge was turned for 

 periods of 4, 5, 7, 9, 10, 11, 12, 13, 14, and 15 seconds. At the ex- 

 piration of each test, the eggs were removed and examined under a 

 microscope. That number of seconds which caused movement of 

 the granules for a given distance could then be recorded. 



For some materials, when relatively high centrifugal speeds are 

 used, the eggs may be thrown too violently against the end of the 

 tube, and injury can result. To prevent such injury, the ends of the 

 tubes may be filled with a small drop of a liquid of specific gravity 

 somewhat higher than that of the cells being centrifuged. For ex- 

 ample, w^hen marine eggs are studied, it is convenient to place in the 

 ends of the tubes a solution of sucrose approximately isotonic with 

 sea water. A 0.95 molal solution is frequently used for this purpose. 

 For nonmarine material, such sugar solutions are too hypertonic, and 

 solutions of gum arable can be used to cushion the cells. It is a re- 

 markable fact that cells may be exposed to strong centrifugal force 

 without injury. Some marine egg cells after fertilization will con- 

 tinue to develop while being vigorously centrifuged. Cells not sur- 

 rounded by a stiff membrane when centrifuged vigorously for long 

 times may separate into halves as a result of the opposing pulls of 

 light and heavy granules moving in opposite directions. But, luitil 

 such breaking occurs, the cells behave normally. 



With the centrifuge method it is a simple matter to obtain relative 

 viscosity measurements. The protoplasmic viscosity under one set 

 of conditions is compared to the viscosity under another set of condi- 

 tions. Measurements of absolute viscosity are much more difficult. 

 For such measurements it is necessary to know the specific gravity of 



