WALLACE O. FENN 449 



per cent of the particles, which settle only half this distance, are 

 removed each time. 



The suspension as kept permanently is then represented by the 

 area ACDB. To prepare a uniform suspension the whole is then 

 shaken up, allowed to settle, and the upper layer of the suspension 

 pipetted off at such a point that only particles of velocity smaller 

 than Vg are obtained. The final suspension is then represented by 

 the area VgCD, and the average velocity must be measured by the 

 perpendicular bisector of this area. Since VsCD is small compared 

 to the whole it may be neglected, particularly as Vs is necessarily 

 more or less indeterminate in practice. The average V is, then, best 

 given by the bisector of the triangle VgVsD or Vav which can be shown 

 by elementary geometry to have the value given in equation (2) 

 above. 



Stokes's law (4) gives the velocity, V, in centimeters per second of 

 a spherical body of radius, r, and density, D, in a medium of absolute 

 viscosity, 77, and density, d, as 



^^ 2 {D-d) , 



V = gr^ 



9 ri 



whence 



■V 



9 Vv 



2g {D - d) 



T 4. J • 1 0.010 erg second - ,^. , ^^. centimeter , 



Introducmg values ^ for 77 (5) and 981 for 



centimeter* . second^ 



g, and changing V into velocity in and r from centimeters 



hour 



to microns we have 



4.5 V 0.01 X 10» 

 981 {D - d) 3,600 



and 



-i 



Diameter = Ji-^AJL (3) 

 J{D-d) 



For the purposes of these experiments it is possible to use this 

 formula directly without allowing for the differences in the density 

 and viscosity of the phagocytic mixtures. The viscosity has no 

 effect on the relative chances of collision because it modifies the 



