146 



method, the presence of Planosarcina ureae in various Dutch soils 

 could easily be demonstrated. 



The discussion of Beijerinck's memoir on the urea bacteria would 

 be incomplete, if no reference was made here to the elegant and 

 simple procedure which he devised as a quick test of urea-decompo- 

 sing ability applicable to various microbes, or to vegetable and animal 

 tissues. It is sufficiënt to place some of the material to be tested on 

 the surf ace of a gelatine plate which contains 1 2 per cent of gelatine, 

 yeast extract, and2or 3 per cent urea. If the test material converts the 

 urea into ammonium carbonate, one observes after a few minutes in 

 the surface of the gelatine directly surrounding the test material a 

 very fine precipitate, formed initially in a very thin layer. On looking 

 at the plate at a certain angle the precipitate manifests itself clearly 

 by the formation of beautiful Newton diffraction rings, Beijerinck 

 has given the name of "iris-phenomenon" to the effect. It is easily 

 shown that the phenomenon is primarily due to the formation of am- 

 monium carbonate by the bacteria, the direct application of the said 

 salt giving at once the same effect. The precipitate ultimately formed 

 is probably a mixture of calcium carbonate and calcium phosphate i ) ; 

 for some reason or other the precipitation begins at the surface of the 

 gelatine gel. 



Thanks to this very sensitive, yet simple reaction, Beijerinck was 

 in later years able to demonstrate the presence of urea-decomposing 

 power in several strains of root nodule bacteria 2). The significance of 

 this finding has not yet been elucidated. 



p. Bacillus oligocarhophüus, an agent of the hiological purification 

 of the air. 



At some time Beijerinck observed the development of a quite 

 specific microflora in a medium which only contained small quantities 

 of nitrate, phosphate and traces of salts of magnesium, manganese 

 and iron. This surprizing phenomenon led to a careful study, made in 

 collaboration with his assistant van Delden, the results of which 

 were published in 1903 3). Since the experiment had been performed 

 in the dark, and, therefore, light was not an energy source, the problem 

 arose at once from where the energy necessary for the development of 

 this flora originated. If such a source could be indicated it would be, of 

 course, possible to ascribe the origin of the organic material, ac- 

 cumulating in this inorganic medium, to a reduction of the carbon 

 dioxide of the air. It is well known that the nitrifying bacteria, for 

 example, are able to couvert carbon dioxide into cell material with the 



1) The yeast extract always contains a small amount of soluble calcium salts. 



2) Nature 112, 439, 1923. 



3) M. W. Beijerinck und A. van Delden, Centralbl. f. Bakt. u. Parasitenk. II, 

 10,33, 1903. 



