242 Dr. H. Chick. Tlie Process of Nitrification [Apr. 1, 



oxidation to nitrate but cannot act upon ammonia, being indeed inhibited in 

 its development by minute traces of that substance. Winogradsky, by him- 

 self, and in conjunction with Omeliansky,* subjected these bacteria to a very 

 exhaustive study. The most striking characteristic that they demonstrated 

 was the marked repugnance of both forms to organic substances. Not only, 

 in opposition to the rest of the plant world, do these organisms make no 

 nutritive use of sugars, peptones, etc., but the presence of more than a trace 

 of such organic substances was found to entirely inhibit their development, 

 thus explaining the failure of all attempts to isolate these bacteria by using 

 the ordinary nutrient culture-media. Winogradsky, on the other hand, had 

 succeeded in cultivating them by employing a silica-jelly-medium impregnated 

 with inorganic salts, and a total inability to grow on organic nutrient media 

 was afterwards put forward by him as a definite practical criterion of the 

 purity of cultures of nitrifying bacteria. This criterion has been challenged 

 by Burri and Stutzer/f by Stutzer and Hartleb,t and later by Fremlin.§ 

 It has been shown by Winogradsky,|| and also by Gartner, Frankel, and 

 Kriigerlf that the former workers were misled by an admixture of non- 

 nitrifying organisms. The most recent work, that of Eoulanger and 

 Massol ,** and of Wimmerff confirms Winogradsky 's criterion. 



The special case of nitrification considered in this paper is that occurring 

 during sewage purification, which aims at the complete oxidation and 

 mineralisation of putrescible substances present. Nitrification is here of great 

 importance, and the effluent of perfectly-treated sewage should contain all its 

 nitrogen in the form of nitrates. 



Although land-treatment of sewage is theoretically the most economical, 

 yet artificial processes, by which space can be saved, have often to be employed. 

 Two processes concern us here, both involving the use of " filter-beds " of 

 coke or other porous material, in which the sewage, usually after having 

 been treated in a " septic tank " is oxidised by bacteria. (1) Contact Filters. 

 — In these the filter-bed is first entirely filled up with the liquor and then 

 after a time allowed to empty slowly, and finally to remain empty for a 

 period. This cycle usually occupies about eight hours, and often may have 

 to be repeated before the effluent is sufficiently purified. (2) Continuous 



* Winogradsky and Omeliansky, 'Centralbl. f. Bakt.,' 2 abt., 5, 1899. 



t Burri and Stutzer, 'Centralbl. f. Bakt.,' 2 abt, 1 and 2, 1895 and 1896. 



% Stutzer and Hartleb, ' Centralbl. f. Bakt.,' 2 abt., 2 and 3, 1896 and 1897. 



§ Fremlin, 'Journal of Hygiene,' vol. 3, 1903. 



fl Winogradsky, ' Centralbl. f. Bakt.,' 2 abt., 2, 1896. 



IT Gartner, Frankel, Kriiger, 'Centralbl. f. Bakt.,' 2 abt, 4, 1898. 

 ** Boulanger and Massol, 'Ann. de l'lnst Past.,' vol. 17, 1903. 

 ft Wimmer, ' Zeitschr. f. Hygiene,' vol. 48, 1904. 



