226 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



of nitrification. A very interesting result was also demon- 

 strated by these experiments, viz., that the nitrification was 

 a progressive phenomenon, and its course was considerably 

 interfered with if, after it had once been established, the 

 sequence of the trays was altered, the last tray, e.g., being 

 substituted for the highest, in which case the nitrification 

 was considerably impeded, until the original conditions were 

 re-established. 



The bacteriological conditions obtaining in sewage filters 

 of this description have been worked out in recent years by 

 Boullanger and Massol at the Pasteur Institut at Lille, by 

 Schulze-Schulzenstein in Germany, and by Dr. Harriette 

 Chick of the Lister Institute. All these investigators agree 

 that the nitrifying organisms found in ordinary sewage filters 

 are the same as those which occur in soil. Boullanger and 

 Massol have found an explanation for the seeming discre- 

 pancy between the results of Winogradski and those which 

 are obtained on sewage filters. According to Winogradski, 

 it will be remembered, the activity of the nitrifying organism 

 is inhibited by the presence of ammonia or of organic matter. 

 Boullanger and Massol concluded from their experiments, that 

 while the presence of large quantities of ammonia or of 

 organic matter may impede the original development of the 

 nitric organism, yet if the growth of this is once established, 

 its activity is unaffected by these conditions. These results 

 are in harmony with Adeney's conclusion that the presence of 

 peaty matter is of assistance in maintaining the activity of the 

 nitric organism. In a sewage filter the extended surface 

 enables an abundant growth of nitrifying organism to take 

 place ; at the same time it is well known that if the maxi- 

 mum load, as it were, of sewage matter is put upon the 

 filter in its early stages, before nitrification is established, 

 it is difficult, if not impossible, for the right conditions to 

 be set up later. It is consequently necessary to ' ripen ' 

 the filter, by putting on only comparatively small quantities 



