THE NITRITE-BACTERIA 189 



Beijerinck gives the following prescription for the preparation of 

 agar which will not inhibit the growth of these organisms. The agar 

 is dissolved in distilled water, filtered, and then allowed to remain 

 as a solid layer under water for two weeks. This sets up putrefaction 

 in the agar, the superimposed water becomes turbid, and a somewhat 

 nasty odour is emitted. This water is poured off and replaced by 

 a fresh supply. After another two weeks the agar will have been 

 sufficiently purified for the purpose. To it are added 0'2 per cent, of 

 sodium ammonium phosphate, 05 per cent, of calcium chloride, and 

 0'05 per cent, of chalk. On the plates of the nutrient agar prepared 

 in this way colonies do not appear for three or four weeks after 

 inoculation. A quicker growth can be obtained by using Wino- 

 gradsky's Silicon jelly as the solidifying medium. The preparation 

 of the nutrient medium of which silicon jelly forms a part is somewhat 

 complicated, and we cannot enter into the details here, but in its 

 composition are also found the following substances : ammonium 

 sulphate, potassium phosphate, magnesium sulphate, ferrous sulphate, 

 sodium chloride, and magnesium carbonate. 1 



. 3. VARIETIES OF NITRITE-BACTERIA. 



It has already been stated that the first of these bacteria was found 

 in Zurich. The same variety has been isolated from the neighbour- 

 hood of Paris, and has since been found to be quite common over the 

 whole of West Europe. The variety that was -found in St. Petersburg 

 differs from the above in being quite round and somewhat smaller, 

 being about 1 //, in diameter and so far as yet observed immotile. 

 Otherwise the life history of this variety is identical with that of the 

 above-mentioned forms. 



A variety of the same kind of bacteria is a small coccus 0'5-0'6/x. 

 in diameter, and provided with a cilium 30 \i in length. The life- 

 history of this organism, which was obtained from Java, agrees in 

 all respects with that of the European varieties, though found at 

 the other side of the globe. Fig. 115 shows a Zoogloea mass of this 

 variety. This has an unusually thick texture ; so much so that the 

 contained individuals cannot be distinguished even with the aid of 

 stains. The free individuals in the motile condition possess each 

 a long cilium, which causes the individual to exhibit a slow, hovering 



1 A detailed method of preparation of this nutrient medium is given in Lafar's 

 Handbuch der technischen Mycoloyie. Vierte Lieferung. 



