214 PROTOPLASM 



It has been shown that bacteria may be centrifuged at high 

 speeds and yet Uve. On the other hand, A. R. Moore finds that 

 sUme mold protoplasm may be severely injured and growth 

 retarded by centrifuging. The problem of possible injury 

 caused by a method of investigation, and the resulting patho- 

 logical changes that may occur is one to which most methods of 

 determining protoplasmic consistency are subject. 



A further limitation of the centrifuge method (but by no 

 means peculiar to it) is the impossibility of ascertaining with 

 any exactness the values needed for computing viscosity— in 

 this case, from Stokes' formula— such as the density of the 

 dispersion medium (the protoplasm) and the density and radius 

 of the suspended particles, which are mere specks under the 

 highest powers of the microscope. Any estimation of these 

 values is likely to involve a large error. There is also the ques- 

 tion whether or not Stokes' law or any of the single constant 

 laws of true viscous flow (of Newton, Maxwell, et at.) based 

 on pure fluids hold in the case of protoplasm any more than 

 they do for gelatin and other substances which show anomalous 

 behavior (page 227). This question is probably best answered, 

 as it is in the case of the applicability of the gas laws to solutions, 

 by regarding the laws as fairly accurately applicable at low 

 dilution, i.e., low viscosity, but not at high concentration, i.e., 

 high protoplasmic consistency. When the centrifuge method is 

 appUed to thin protoplasm, it probably gives fairly accurate 

 values, which have the great advantage of being capable of 

 experimental reproduction. L. V. Heilbrunn has applied the 

 centrifuge method to studies of protoplasmic consistency. 



The Brownian-movement Method. — Brownian movement 

 is the zigzag motion exhibited by all colloidal particles when 

 suspended in a medium of not too high consistency (less than 

 that of glycerin). The smaller the particle and the thinner 

 the medium the greater the motion; therefore, the rate of the 

 Brownian movement of included particles is an indication of 

 the degree of viscosity of the medium. But this is accurate 

 only for pure liquids and true solutions. In polyphasic systems 

 such as protoplasm, the movement of particles in one of the 

 phases does not necessarily throw light on the state of the mass 

 as a whole; thus. Mast and others have shown that there may 

 be very active movement of particles in highly viscous proto- 



