242 ELEMENTS OF WATER BACTERIOLOGY 



who wishes to follow out this line of investigation 

 will find a good summary of what is already known 

 of the hydrolysis and denitrification of nitrogenous 

 bodies and the decomposition of cellulose and other 

 carbohydrates in Rideal's " Sewage and the Bacterial 

 Purification of Sewage " (1906). 



Gage (1905) has made a suggestive study of the 

 bacteria which carry on the reducing changes in sewage 

 which deserves the attention of all who are interested in 

 the more theoretical aspects of sewage treatment. 

 His method consisted in plating sewages and effluents, 

 isolating typical cultures and determining their power 

 to decompose peptone and nitrates with the produc- 

 tion of ammonia and free nitrogen. The rate of gelatin 

 liquefaction, the amount of nitrate reduced, the amount 

 of free ammonia formed, and the amount of nitrogen 

 liberated were quantitatively determined for each culture 

 thus isolated. The numerical values obtained, multiplied 

 by the number of bacteria, apparently of the same type, 

 observed in the plates, gave coefficients of the liquefying, 

 denitrifying, ammonifying, and nitrogen-liberating power 

 of the effluent; 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 



