298 BACTERIOLOGICAL AND EKZYME CHEMISTRY 



by the filtering medium. Unbroken down albuminoid matter 

 is further peptonised, and ammonia is oxidised to nitrite 

 and finally nitrate. Fatty acids and other carbonaceous 

 matters are finally oxidised to COj. This can be verified, if 

 the gases in the interior of such a bed are drawn off and 

 analysed, when a marked increase in the CO2 over that present 

 in the atmosphere will be noticed. Moreover, if a portion 

 of the filtering material is carefully removed from the bed 

 without disturbing its coating of slime and is placed in a closed 

 vessel provided with a manometer, an appreciable rise in the 

 mercury may be observed, owing to the absorption of the 

 oxygen in the containing vessel. The presence of nitrates can 

 be determined by washing the material with water free from 

 nitrate, and testing for the presence of the latter in the 

 washings. 



The changes just described take place while the bed is 

 standing empty, and are characterised by the predominance 

 of nitrification ; when the bed is again filled with liquid a 

 somewhat difEerent set of conditions arises. Mechanical 

 absorption of the more insoluble matter takes place as 

 already described, but oxidation also occurs, through inter- 

 action of the nitrates present with these substances and with 

 impurities present in solution ; in this way finely divided 

 cellulose may be finally oxidised, as was explained in 

 Chapter X. During these changes, which may be grouped 

 together as de-nitrification changes, loss of nitrogen occurs, as 

 has been shown in Chapter XIIT, either as free nitrogen or, 

 it may be, as nitrous oxide, NjO, this gas having actually 

 been discovered by Letts in solution in the liquid contents of 

 a contact bed. 



The proper working of a contact bed can be controlled, by 

 having regard particularly to the amount of nitrate present in 

 the effluent, especially in the first discharge after a long period 

 of rest. The nitrates present represent the overplus left after 

 de-nitrification has taken place ; within limits, the longer the 



