872 Mockeridge . — Some Conditions influencing the Fixation of 
best organic constituent having been accepted for the time being as mannite. 
Mine Krzemieniewski has shown that potassium, calcium, sulphur, magne- 
sium, and phosphorus are all essential to the development of Azotobacter . 
Recent work by Omeliansky and Ssewerowa, and also by Remy and Rosing, 
has brought out the importance of iron, while Kaserer lays stress on the 
need for aluminium and silicon also ; but further reference will be made to 
this work later. 
The solutions, apart from those containing iron, aluminium, and silicon, 
which had been used with more or less success, practically resolve them- 
selves into two classes: firstly, those in which neutralization of the acid 
potassium phosphate had been effected simply by the addition of excess of 
calcium carbonate ; and, secondly, those in which the acidity had been 
neutralized by the addition of dilute sodium-hydrate solution, either to the 
whole culture solution, or to a solution of the phosphate made separately 
and afterwards added to the main bulk, the necessary calcium being 
supplied to the medium in the form of a small amount of a salt other than 
the carbonate. A combination of the two methods is found in the medium 
used by Ashby, who dissolves the phosphate separately, just neutralizes 
it with sodium hydrate, and, after the addition of this to the main portion 
of the solution, adds an excess of calcium carbonate. The solution used by 
Beijerinck consisted of : 
Tap water .... 1,000 c.c. 
Mannite .... 20 grm. 
Di-potassium phosphate . . o-2 grm. 
and, having used this for purposes of experiment, he claims that Azoto- 
bacter will not, in pure culture, fix nitrogen. Chester points out that the 
medium is at fault, since no attempt is made to correct the acidity of the 
phosphate, and he reports that a good fixation of nitrogen has been 
obtained at the New Jersey Experiment Station on a medium consisting 
of the following : 
Tap water .... 1,000 c.c. 
Mannite .... 15 grm. 
Di-potassium phosphate . . 0-5 grm. 
Magnesium sulphate . . o-2 grm. 
Calcium chloride . . . 0-02 grm. 
the whole being rendered alkaline by means of sodium hydrate. Many 
attempts have been made by the writer to obtain growths of Azotobacter 
chroococcum on this medium, with very little success ; and when a surface 
film has developed at all, it has been very scanty, the best having been 
obtained in solutions which had been rendered just neutral by sodium 
hydrate, the presence of any excess, even in very small amounts, of the 
latter proving fatal to development. No determinations of the nitrogen- 
