THE ABSORPTION OF NITROGEN 199 



finally worked out in 1899 by Miss Dawson, studying in 

 Marshall Ward's laboratory. 



Meanwhile several investigators had been following 

 the line of research opened up by Berthelot on the fixation 

 of free atmospheric nitrogen by bacteria in the soil, but 

 I must leave you to study the details of these investigations 

 in the textbooks, singling out perhaps one name, that 

 of Winogradski, to whose labours we owe much of our 

 knowledge of the remarkable chemical and biological 

 activities of which the soil is the scene, all concerned in 

 the presentation of combined nitrogen to the plant in 

 such forms as may be most readily appropriated by it. 

 Warington at Rothamsted also contributed substantially 

 to the solution of this very difficult problem. One 

 remarkable fact was made clear by his researches, viz. 

 that the activity of the nitrifying organism is retarded 

 by the presence of organic nutrients. Winogradski 

 followed up this discovery by cultivating the bacteria in 

 a sterilised solution of ammonium sulphate, potassium 

 phosphate, and basic magnesium carbonate. In such a 

 solution he found that, in a few days, all the ammonia 

 had been replaced by nitrate and that a sediment was 

 formed composed of bacteria in a zoogloea, which was 

 capable of inducing rapid nitrification in any fluid con- 

 taining ammonia. Winogradski also found that the 

 nitrification proceeded in two stages, first, the formation 

 of a nitrite and then of a nitrate. Further research 

 showed that there were two organisms concerned, one 

 transforming ammonia into nitrite and a second trans- 

 forming nitrite into nitrate. Both of these obtained 

 their carbon from the carbon dioxide of the air or from 

 carbonates in the solution, but in neither case from 

 organic compounds of carbon. Later on, in 1890, 

 Winogradski showed that it was the carbon dioxide only 

 that was the source of the carbon. This constructive 

 metabolism was independent of light and, of course, of 

 chlorophyll, and hence was termed chemosynthesis 



