84 BACTERIOLOGICAL EXAMINATION OF WATER. 



suspended in a little distilled water and sterilised by boiling, 

 was added. A small drop of a recently nitrified liquid placed in 

 a flask produced marked nitrification, and at the expiration of 

 fifteen days all traces of ammonia had disappeared. The oxida- 

 tion was found to be produced by zoogloeae of bacteria which 

 formed on the surface of the magnesium carbonate at the bottom 

 of the flask. The bacteria which appeared on the surface of the 

 fluid in the flask had no nitrifying action. 



In later experiments Winogradsky prepared an absolutely pure 

 sulphate of ammonium and employed calcium carbonate instead 

 of magnesium carbonate. Portions of the zooglcea mass dis- 

 tributed over gelatine plates produced no growth, but the deposit 

 of crystals of calcium carbonate introduced into flasks containing 

 the salt solution produced marked nitrification after the lapse of 

 three weeks, the delay in the oxidation process being caused by 

 the small quantity of the nitrifying organisms placed in the 

 flask. In order to obtain plate cultures of nitrifying organisms 

 Winogradsky employed a silica which he prepared in the follow- 

 ing manner : Commercial sodium silicate was diluted with three 

 times its volume of water and 100 c.c. of the dilution added to 

 50 c.c. of dilute hydrochloric acid, the mixture was then dialysed. 

 The dialyser was kept in running water for twenty-four hours 

 and then in distilled water, frequently changed, for two days. 

 The solution so obtained was placed in a flask plugged with 

 cotton wool and sterilised. The following solution of salts was 

 required to produce gelatinisation : Sulphate of ammonia 0'4 

 gramme, sulphate of magnesium 0*05 gramme, phosphate of 

 potassium 0*1 gramme, chloride of calcium a trace, carbonate of 

 soda 0'6 to 0'9 gramme, distilled water 100 c.c. The two 

 sulphates with the chloride and the phosphate with the 

 carbonate were dissolved and sterilised separately, and then 

 mixed when cool. Small glass dishes, having a diameter of 5 

 centimetres, were employed for the preparation of plates. The 

 solution of silica was evaporated to about half its volume, or 

 until two or three drops mixed with one drop of the saline 

 solution produced gelatinisation in about five minutes. The 

 silica solution was then placed in the glass dishes by means of a 

 pipette and one-third to one-half its volume of the salt solution 

 added. Sometimes magnesium carbonate was used instead of 



