M. A. LAUFFER 



The Einstein equation has been subjected to a rigorous test by 

 Eirich, Bunzl, and Margaretha (8). These investigators used rotating 

 cylinder, falling ball, and capillary viscometers to measure the vis- 

 cosity of glass spheres with a radius of 80 microns suspended in a 

 solution of mercuric nitrate in nitric acid, mushroom spores with a 

 radius of 4 microns dispersed in a mixture of olive oil and tetrachloro- 

 ethane, and yeast cells with an average radius of 2.5 microns dispersed 

 in water. The spherical natures of all of these particles were ascer- 

 tained by examination with the microscope. The intrinsic viscosities 



Table I 

 Viscosity of Yeast Suspensions 



obtained in every case differed from that given by the Einstein equa- 

 tion, 2.5, by less than 10%. This means that the Einstein equation 

 is reliable when it is applied to systems which conform to the conditions 

 for which it was derived. Representative of the data of Eirich and 

 collaborators are the results presented in Table I, where the specific 

 viscosities obtained in a capillary viscometer of a yeast suspension at 

 various concentrations are compared with specific viscosities calculated 

 from the simple form of the Einstein equation. It can be seen that the 

 equation fits the data in very dilute suspensions. 



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