286 



Trans. Acad. Sci. of St. Louis. 



bation, the reaction of the medium being + 1.5%, and 12% 

 gelatin being used in each case. For some reason in this 

 series, the samples from Lockport, Joliet and Peoria showed 

 few liquefying bacteria on the third day of incubation, while 

 those from Alton and lower down along the stream showed 

 not only more liquetiers, but the liquefaction of the gelatin 

 had usually progressed to a considerable extent. At a cer- 

 tain period of the work I was stationed at Joliet, 111., and 

 samples from that district could be plated soon after collec- 

 tion. In these cases, I found a large percentage of liquefying 

 colonies so that the low averages as shown in the table may 

 be explained by the condition of transportation of the sample. 

 The quantitative estimation as shown by plate cultures 

 should always be considered as relative and not as giving the 

 absolute number of bacteria, for it has been found that 

 various media used for plating give widely different results. 

 The nature of the nutrient media, the conditions of growth, 

 the presence or absence of air, moisture, etc., are all im- 

 portant factors modifying the actual bacterial number. To 

 demonstrate what effect the absence of air would have on the 

 number of bacteria, plates were prepared in the regular way 

 and similar sets of plates were grown under anaerobic condi- 

 tions with the following results : — 



TABLE X. 



With the removal of oxygen but half the number of 

 bacteria developed, and it is important to consider this fact in 

 connection with sewage polluted water. The quantitative 

 examinations, while not giving absolute results, are often of 

 value when the numbers run very high as an indication 



