NITEIFICATION 187 



from growing, and among these the nitrification bacteria were pre- 

 dominant. By an ingenious device the Russian bacteriologist succeeded 

 in effecting a pure culture, so another step in our knowledge of this 

 subject was achieved. Now, when this organism was inoculated into 

 a sterilised ammoniacal nutrient, solution, it was found that it was 

 capable only of changing ammonia into nitrite ; no subsequent change 

 of the latter into a nitrate took place when pure cultures were used. 

 The discovery of this fact showed that the process of nitrification is 

 not performed wholly by one kind of organism, there must also be 

 present in the soil another kind which changes the nitrite into nitrate. 

 It was not long before the genius of Winogradsky enabled him to 

 isolate from the soil an organism which effected this change, so that 

 now the essentials of the whole process can be regarded as complete. 

 These bacterial organisms isolated by Winogradsky and others may 

 all be classified under one or other of the two heads : the nitrite- 

 bacteria which ■ oxidise ammonium compounds into nitrites, and 

 nitrate-bacteria which still further oxidise the nitrites into nitrates. 



§ 2. THE NITRITE-BACTERIA. 



The first of the nitrite-bacteria was isolated by Winogradsky from 

 a specimen of soil which came from Zurich (Switzerland). This 

 organism was named first Nitromonas, this being subsequently changed 

 into Nitrosomonas. It is important to note that the name applies 

 not so much to a single species as to a group 

 of very closely allied organisms, the differences 

 between which being so small that it is not 

 advisable to give each a separate name. The 

 variety found at Zurich is very widely dis- 

 tributed, and is practically the only one of 

 this kind that is found in West Europe. 

 The individuals are extremely small, and do fio- us.— zoogioeae from 



■^ nitnte-bacteria. (Zurich.) 



not create turbidity in the culture fluid, as 



is done by the majority of bacteria. In fact they are not usually 

 found in the liquid, but rather in the sediment as compact Zoogloeae 

 of various sizes, each of the latter being round, and measuring from 

 10;u.to 15/i in diameter (Fig. 113). In order to see the individuals 

 which are embedded in a Zoogloea, careful staining is necessary, the 

 best for the purpose being a solution of iodine in potassium iodide. 

 A 7-10 days' old culture shows, when undisturbed, either a very 



