202 Elements of Water Bacteriology. 



ammonjfyipg, and nitrogen liberating power of the efflu- 

 ent; and these coefficients may be considered as measures, 

 for a given sample, of the tendency of the bacterial 

 flora to set up certain changes. The results of further 

 studies made by Clark and Gage (1905), on sewages and 

 on sand, contact, and . trickling effluents, show that there 

 may be important differences between various sewages in 

 this respect which must render their purification more 

 or less easy. They indicate that the effluents obtained 

 from intermittent sand filters in cold weather contain 

 larger numbers of ammonifying and denitrifying bacteria 

 than appear at other seasons, which may help to explain 

 the poorly nitrified effluents obtained in the winter season. 

 Along these and similar lines research work in sewage 

 bacteriology promises to be fruitful of results. 



