1916] 
DUGGAR AND DAVIS—NITROGEN FIXATION 429 
more sodium thiosulphate (3 gm. instead of 2) and to allow 
10-15 minutes after its apparent decomposition had taken 
place before digestion was continued. This illustrates the 
possibility of error in a method that is not thoroughly tested 
in connection with the peculiar conditions at hand. 
No difficulty was experienced in obtaining results with the 
Gunning-Kjeldahl modification that checked within experi- 
mental error. Some trouble, through frothing, will be ex- 
perienced in the actual digestion, however, where the culture 
media are high in sugars. This may be overcome by boiling 
(after adding 15 ee. of concentrated sulphuric acid) slowly 
for an hour or more, then adding more acid together with 
15 gm. of potassium sulphate and 1 gm. of mercury. In 
our work it became necessary at times to add a third lot of 
15 се. acid—the same amount being always added to both 
fungus-containing flasks and controls. 
Distillation was carried out through block tin tubes which 
had been in use sufficiently long to obviate the possibility of 
error through absorption of ammonia—a point observed with 
new tin by several investigators. The standard acid and 
alkali were restandardized at short intervals. The same lot 
of chemicals was always used throughout a single series to 
insure parallel treatment with both fungus-containing flasks 
and controls. 
EXPERIMENTAL RESULTS AND DISCUSSION 
The results of our experiments are presented in some de- 
tail in table т. It is necessary to note that while the quantities 
given in column v were obtained by careful weighings, they 
represent only approximately the quantities present in the 
solution as determined by analysis (see columns vi and vir). 
In any series the controls are as nearly perfect as we were 
able to arrange, that is, the solutions in all the flasks in any 
one series were taken from a single vessel of the culture 
medium, the complete mixing of the different constituents in 
the culture solution being given special attention. In column 
1 the letters A and B given in connection with the series num- 
bers refer to the two nutrient salt solutions employed as de- 
