448 



PHAGOCYTOSIS OF SOLID PARTICLES. 



the same rate leaving a clear liquid above. In all the suspensions 

 used in these experiments this was possible. From the velocity so 

 obtained, the diameter can be calculated by Stokes's law. 



For example, if a suspension has been allowed to settle for 1 hour 

 until 5 cm. of clear water remain at the top, the upturned capillary 

 tip of a siphon is now placed 7 cm. below the surface, and the upper 

 2 cm. of the suspension are drawn off, the velocity of these particles 

 is somewhere between 5 and 7 cm. per hour. It is probably given 

 most accurately by the formula 



Ve - Vs 



Vav = Fg - 



V'2 



^ (2) 



where Vs and Vg are the smallest and largest velocities, respectively. 

 This may be seen from a consideration of Fig. 3. Here the abscissae 

 represent velocities. Ordinates represent the number of particles of 



Fig. 3. Ordinates represent number of particles, abscissae represent velocities 

 or diameters squared. Original raw material is represented by AONB, N par- 

 ticles of each velocity being assumed. Dotted lines show portions removed after 

 ten successive settlings. ACDB is the permanent suspension before large par- 

 ticles are removed. Vg CD is, then, the frequency curve of a uniform suspension 

 removed from the top as suspension ACDB settles. Vav is the average velocity. 



each velocity originally present; it is assumed that there were the 

 same number, N, of each in the original raw material. The area, 

 AONB, then represents the original suspension. The dotted lines 

 DO, etc., outline the parts of the suspension which were successively 

 discarded with the supernatant hquid after being allowed to settle 

 ten times until particles of velocity, Vs, had just settled out. 50 



