454 BACTERIOLOGY. 



TABLE ILLUSTRATING THE PROPORTION BETWEEN THE RE- 

 SULTS OBTAINED BY THE USE OF GELATIN AND AGAR-AGAR 

 IN QUANTITATIVE BACTERIOLOGICAL ANALYSIS OF WATER. 

 RESULTS RECORDED ARE THE NUMBER OF COLONIES THAT 

 DEVELOPED FROM THE SAME AMOUNT OF WATER IN EACH 

 SERIES.* 



NUMBER OF COLONIES FROM WATER THAT DEVELOPED UPON 



Gelatin plates at 18 to 20 C. Agar-agar plates at 37 to 38 C. 



310 ........ 170 



280 ...>.... 140 



310) fl80 



340 * 1 160 



6501 f210 



630 F 1 320 



380 ) f 290 



400) ' 1 210 



10001 (100 



890) 1 130 



340! f280 



370 < 1210 



490 ) f 110 



580 S 1 100 



Throughout this part of the work it is to be borne 

 in mind that when one refers to plates it is not to a 

 set, as in the isolation experiments, but to a single 

 plate. 



METHOD OF COUNTING THE COLONIES ON PLATES. 

 For convenience in counting colonies on plates or in 

 tubes, it is customary to divide the whole area of the 

 gelatin occupied by colonies into smaller areas, and 

 either count all the colonies in each of these areas and 

 add the several sums together for the total ; or to count 

 the number of colonies in each of several areas, ten or 

 twelve, take the mean of the results and multiply this 

 by the number of areas containing colonies. 



1 I am indebted to Dr. James Homer Wright, Thomas Scott Fellow in 

 Hygiene (1892-93), University of Pennsylvania, for the results presented in 

 this table. 



