450 PHAGOCYTOSIS OF SOLID PARTICLES. I 



velocity of both the cells and particles equally. Since the density 

 of the medium was only 1.01 this can also be disregarded as small in 

 comparison with the density of the particles, 1.81 (carbon) and 2.68 

 (quartz). As far as the leucocytes were concerned with a density 

 1.11, this was really allowed for largely by the necessity of measuring 

 their velocity in a medium of sodium chloride of density 1.007. 



In the case of suspensions which are not uniform enough to settle 

 with a sharp boundary' it is impossible to determine anything but the 

 velocity of their smallest particles by direct observation. For such 

 cases a method was devised which is known as the "stop-cock" method. 

 It was also necessary to use this method for determining the velocity 

 of the leucocytes. The method consists in allowing the suspension 

 to settle out in a glass tube provided with a specially made stop-cock, 

 such that there was no constriction in the inside diameter; there was, 

 thus, a straight uniform tube all the way through the stop-cock. 

 After a given interval of time the stop-cock is turned. The change 

 of concentration in the suspension above the stop-cock is determined 

 by counts of the numbers of particles present before and after settling. 

 If there are h centimeters of suspension above the stop-cock and if 

 the concentration before and after is given by C and Ci, the velocity 

 is given by the formula 



Ct 



This method gives an average figure for the velocity because the 

 small particles which settle too slowly compensate for the large ones, 

 all of which may have passed below the stop-cock before it is turned. 

 Obviously, however, the diameter which is calculated from the average 

 velocity is not the average diameter because the velocity is propor- 

 tional to the square of the diameter. The agreement between results 

 by this method and measurements of the diameter of particles by 

 microscopic methods was, however, sufficiently good for these pur- 

 poses as shown in Table I. The agreement is in itself proof of the 

 uniformity of the suspensions used. 



In the table are also included some figures for the diameters of 

 these suspensions by the method of evaporating to dryness a suspen- 

 sion containing a known number of particles, and calculating the 



' This is the case with suspensions from which the large particles were removed 

 by centrifuge instead of by settling. 



