392 APPLIEB BACTERIOLOGY 



unusual disturbance in the filter, and should make the 

 superintendent more attentive to the possible causes of it. 



Every city in the German empire using sand-filtered 

 water is required to make a quarterly report of its working 

 results, especially of the bacterial character of the yrater 

 before and after filtration, to the Imperial Board of Health. 



Detection of Sewage Pollution. — With reference to the 

 general question of the bacteriological examination of 

 drinking-water, much information as to the character of 

 a water is gained by incubating a small quantity of the 

 sample at blood-heat for twenty-four hours. The number 

 of organisms is then ascertained by an ordinary gelatine 

 plate culture. The number of organisms so found is com- 

 pared with the number of organisms found by a direct 

 gelatine plate culture, which is made on the water imme- 

 diately upon the receipt of the sample. If a sample of 

 water is polluted with sewage, a great increase in the 

 number of the organisms will be found to have taken place 

 as the result of the incubation. All the organisms normally 

 present in faeces grow and multiply vigorously at blood- 

 heat, whereas this temperature is fatal to the majority of 

 the common water bacteria ; therefore a corresponding 

 . decrease in the number of the organisms will be found to 

 have taken place in a pure water. 



A more convenient plan is to prepare an agar-agar plate 

 culture with a fraction of the c.c. of the water. The result- 

 ing plate is incubated at blood-heat for thirty-six hours. 

 This method is the most satisfactory, as it has the advan- 

 tage that the actual number of the micro-organisms that 

 will grow at blood-heat is ascertained. 



The following results of experiments made by one of us 

 on waters of known origin shows the value of these two 

 methods : 



