WALLACE O. FENN 



451 



diameters on the assumption that each particle is a sphere. It is 

 doubtless due to the irregular sizes of the particles that this method 

 gives results which are uniformly lower than the other methods. At 

 the end of the table are also included some figures for the smallest 

 diameters of the particles as calculated from the velocity with which 

 the upper boundary of the suspension settles under gravity; i.e., 

 the velocity of the smallest particles. This figure can readily be 

 obtained from the stop-cock method in addition to the figure for the 



TABLE I. 



Measurements of Diameters in Microns of Three Sizes of Quartz Particles by Various 



Methods. 



average velocity and gives a good idea of the degree of uniformity 

 of the suspension. To avoid counting, the stop-cock method was 

 usually modified by the use of the colorimeter for comparing the con- 

 centration of the suspension before and after settling.^ All the meas- 

 urements in Table I were made on suspensions prepared separately 



* For this purpose underneath illumination only was used so that the intensity 

 of the transmitted, not the diffracted, beam was measured. The use of the color- 

 imeter is perhaps a doubtful measure on theoretical grounds unless the suspensions 

 are thoroughly uniform, because it assumes that the amount of light transmitted 

 is proportional to the number of particles without respect to size. After settling 

 there are of course relatively fewer large particles. 



