BACTERIOLOGY OF SEWAGE 229 



sources of water-supply the town taking water may 

 protect itself by filtration. It should so protect itself, 

 at any rate, from the pollution necessarily incident to 

 surface waters; and, imless the bacterial condition of a 

 stream or lake is made very materially worse by the 

 discharge of sewage effluents, it is fair that the respon- 

 sibility of purification should rest on the water works, 

 rather than on the sewage purification plant. Shell- 

 fish, on the other hand, cannot be purified. Either 

 pollution must be prevented, or the industrj^ abandoned. 

 Under such circumstances sanitary authorities may 

 rightly demand, as they have demanded at Baltimore, 

 that bacteria, as well as putrescible organic matter, 

 shall be removed in sewage treatment. Under such 

 circumstances the bacterial control of purification 

 plants is as essential as in the case of water filters. 

 Methods of Bacteriological Examination of Sewage and 

 Effluents. In England, considerable attention has 

 been devoted to this subject, and numerous methods 

 have been recommended as furnishing valuable criteria 

 of the bacterial quaHty of sewage effluents. Houston 

 (1902^), for example, suggests various tests involving 

 the use of litmus milk, peptone solution, gelatin tubes, 

 and neutral-red broth, as well as the inoculation of 

 animals. He considers the determination of the num- 

 bers of B. coli and B. sporogenes as of greatest moment, 

 while the identification of streptococci is of value in 

 certain cases and the enumeration of liquefying bacteria, 

 spore-forming aerobes, thermophilic bacteria, and hydro- 

 gen sulphide producing bacteria is of subsidiary impor- 

 tance. Rideal (1906) has recently recommended a some- 



