WALLACE O. FENN 441 



depend upon the size and density of the particles in relation to the 

 size and density of the cells, large particles should be taken up more 

 rapidly than small particles. This was found to be true, and, more- 

 over, a direct proportionality was found to exist between the experi- 

 mental constant, K, and the relative chance of collisions as calculated. 

 It will be seen that this method presents great difficulties as a means 

 of comparing the rate of phagocytosis of different particles and it was 

 finally discarded in favor of a simpler method which will be described 

 in a subsequent paper. The results by the first method possess 

 considerable theoretical interest, however, and the consideration of 

 the chances of collision should be of interest perhaps in opsonic 

 index work where this factor has never been taken into consideration. 

 The technique of handling the quartz suspensions will first be 

 described; then the method of calculating the chances of collision; 

 and the experimental results, using three different sizes of quartz 

 particles. Similar experiments with carbon and comparisons between 

 quartz and carbon will be described in later papers. 



The Suspensions. 



The first requirement for these experiments is to have the particles 

 of as nearly uniform size as possible. Suspensions prepared by Ham- 

 burger's method would not be sufficiently uniform to admit of a calcu- 

 lation of the chances of colHsion. Uniformity can be obtained by 

 centrifugaHzation or settling. The former is quicker but the product 

 is not so good nor so easily controlled. In both methods the principle 

 consists in washing out the particles which are too small. The larger 

 particles may be left in the suspension until a uniform sample is 

 needed, when the suspension is shaken up and the large ones are either 

 centrifugalized out or allowed to settle out by gravity. In the centri- 

 fuge, however, it is quite impossible not to set up currents which 

 carry some excessively large particles to the top. The small particles 

 can, however, be removed in the centrifuge and the sediment can 

 thus be resuspended and allowed to settle out by gravity. It settles 

 with' a sharp line at the top from which a uniform sample can be 

 withdrawn. 



This combined method was the one usually used in these experi- 

 ments. Suspensions of this sort of carbon and quartz have been 



