IV. VISCOSITY MEASUREMENTS 109 



cosity such as are involved in the change from sol to gel and back 

 again can readily be observed. During gelation, the colloidal fluid 

 in a test tube or beaker suddenly stiffens and, as can be seen at a 

 glance, it will no longer flow when the tube or beaker is tilted or 

 turned upside down. But in a cell the protoplasm may change from 

 a very fluid to a very solid state, and yet, as far as one can see with the 

 microscope, there is no visible indication of any change whatsoever. 

 Unfortunately, the literature on protoplasmic viscosity has been 

 badly confused by the reckless and careless statements of workers 

 who have not used standard methods of viscosity determination. It 

 is a simple matter to poke a cell with a microneedle, but, from such 

 poking, it is not possible even to obtain a correct estimate of the order 

 of magnitude of protoplasmic viscosity. With a microdissection 

 apparatus, a cell may be attacked with a fine glass needle. Ordinar- 

 ily, the distensible cell membrane yields to the pressure of the needle 

 so that there is an indentation of the needle into the cell. It is very 

 difficult to determine whether the needle actually enters the proto- 

 plasm or not. If it does enter, it almost certainly causes pronounced 

 injury in whatever region it reaches. The needle is of course con- 

 trolled mechanically, so that one has no sense of the force involved 

 in moving the needle about. Under such conditions the "measure- 

 ment" of viscosity is mere guess work. 



B. VISCOSITY CONCEPT 



People generally distinguish liquids that are thick and heavy from 

 those that are thin or light. The distinction is made not on the basis 

 of density but rather of the ease with which a liquid flows or pours, 

 or on the basis of how easily it may be stirred. Scientifically, liquids 

 that flow or pour easily and offer relatively little resistance to stirring 

 are less viscous or more fluid than those that flow with difficulty and 

 offer marked resistance to a stirrer. As a matter of fact the terms 

 viscosity and fluidity are rather well established in common language. 

 The exact definition of viscosity is not so easy to present. The defini- 

 tion commonly employed is that formulated by Maxwell. A con- 

 venient wording of this definition is quoted by Barr (5) : 



"The coefficient of viscosity of a fluid is the numerical value of the tangen- 

 tial force on unit area of either of two parallel planes at unit distance apart 

 when the space between these planes is filled with the fluid in question and 

 one of the planes moves with unit velocity in its own plane relatively to the 

 other." 



