WALLACE 0. FENN 471 



4. Decrease in activity of cells due to internal changes or changes 

 in environment. 



An increase with time may be due to : 



5. Increase in velocity of cells caused by increased density after 

 ingestion of particles. 



6. Increase in activity of cells by internal changes or changes in 

 environment. 



Of these factors, No. 3 can be predicted by measurements of the 

 particles in the suspension and has no effect if thoroughly uniform 

 suspensions are used. Nos. 4 and 6 are probably neghgible. Nos. 

 1, 2, and 5, then, are the significant factors. No. 1 is presumably 

 the same for each carbon suspension in any one experiment so that 

 it does not vitiate the comparison. Nos. 2 and 5 are small if the 

 number of cells is large. In the majority of experiments there were 

 from one to two particles per cell in the most concentrated suspen- 

 sion, probably never over three, and frequently less than one. Since 

 each cell could take up perhaps five large particles and fifteen small 

 ones they could hardly be appreciably filled up. It seems valid to 

 use the total K, where K is constant, as well as the initial K for com- 

 parison. Usually the total K does not differ from the initial K by 

 more than the experimental error. The latter is, of course, the more 

 inaccurate as it depends upon only two points, while the total K is 

 the average of all points. It is, perhaps, surprising that K should 

 be so constant. This must indicate that the disturbing factors are 

 in such cases comparatively small and that those causing an increase 

 in K partly compensate for those causing a decrease. 



In Table II the results of five such comparisons of rates of ingestion 

 of the two suspensions of carbon are given. The results for three of 

 these experiments are plotted in Figs. 2, 3, and 4. The other two 

 experiments are somewhat less reliable and are included merely for 

 completeness. 



It must be admitted that the agreement between the theoretical 

 initial ratio and the average experimental ratio is better than the 

 variation in the individual ratios warrants. It will be seen that in 

 Figs. 2 and 3 the second prediction is fulfilled as well as the first, for 

 the slope of the curve representing the ingestion of the 4.7^ particles 

 continually decreases, while that of the 3.2/^ particles remains con- 



