BACTERIA DEVELOPING AT DIFFERENT TEMPERATURES 233 



sewage and water filters, is shown by the smaller per cent of samples 

 from the filtered sources which contain bacteria and acid-producing 

 organisms capable of developing at 50 C. Bacteria of these types 

 occurred only in a small percentage of the samples from ponds 

 and shallow wells, and were entirely absent from samples from springs 

 and driven wells. 



Numbers oj bacteria, 30 series. In Table 2 are shown the aver- 

 age numbers of bacteria and of acid-producers obtained with different 

 waters on plates incubated at 20 C., 30 C., and 40 C. From the 

 figures in this table it is seen that the numbers obtained at the dif- 

 ferent temperatures agree in general with the character of the water 

 under examination. The counts obtained at 30 were larger than 

 those at 40 in every instance when dealing with polluted water, 

 but there was little difference in these counts when dealing with 

 water of good quality. This fact, however, would be an advantage 

 rather than otherwise, since the use of 30 counts would give us a 

 much sharper distinction between good and bad waters than would 

 counts at 40, and would allow this distinction to be made in a min- 

 imum time, 24 hours, as compared with two to four days required 

 to obtain the 20 counts. A rapid method of determining a portion 

 of the bacterial content of waters is especially desirable when deal- 

 ing with water filters, such filters being controlled largely on the 

 basis of the per cent of the bacteria in the raw water which they 

 remove and the number of bacteria in the filtered water. From the 

 figures in the table we find the percentage removal of bacteria by 

 the Lawrence City Filter to be 99.5, 99.7, and 96.7, as determined 

 by the counts at 20, 30, and 40 respectively, and the removal 

 of acid-forming organisms to be 99.1, 99.4, and 95.2 respectively. 

 The percentage efficiency of Filter No. 220 was 98.1, 96.0, and 

 93.8 respectively, computed from the total bacteria developing 

 at 20, 30, and 40 C., and 98.8, 98.6, and 97.7 computed from 

 the acid-forming bacteria developing at these temperatures. The 

 removal of bacteria by Filter No. 216 was 91.0, 96.1, and 82.9 

 respectively, and the removal of acid-forming bacteria was 95.0, 

 94.6, and 82.2 respectively, as shown by the figures at 20, 30, 

 and 40 C. In other words the percentage removal of bacteria and 

 of acid-formers, as determined by counts at 20 C. after four days' 



