878 Mockeridge — Some Conditions influencing the Fixation of 
It appears from these figures that while a good fixation is obtained with 
o-2 percent, of slag, an even better one results from the use of 0-4 percent. 
The addition of more than this, however, produces a perceptible, though not 
very considerable deterioration in nitrogen-fixing power, which effect in- 
creases with increase in the proportion of slag until 1 per cent, is reached, when 
the presence of this quantity appears to stimulate the organisms to greater 
activity than does the use of o-6 or o*8 per cent. The proportion of 0-4 
per cent, however, obviously produces the most beneficial effects, probably 
on account of the stimulative effect of certain constituents of the slag ; the 
addition of these substances in larger quantities than are contained in 0-4 
per cent, being not only unnecessary, but harmful to the organism. The 
results obtained by the use of 1 per cent, of slag occasioned considerable 
surprise, but it seems possible that in this quantity just sufficient of another 
constituent, present in the slag in lesser proportion, has been added, to 
exert a second tonic effect, and overcome, to a certain extent, the depress- 
ing influence of over-doses of those elements to which the first stimulation 
was due. In order to test the truth of this hypothesis and to ascertain the 
effect of still larger proportions of slag, further series of experiments are 
now in progress, the results of which will be given as soon as possible. 1 It 
is clear, however, that the effect of the slag is not limited, as is that 
of calcium carbonate, to a simple neutralization of acidity, but that certain 
constituents, though not essential to the growth of and fixation of nitrogen 
by Azotobacter , yet when added in the correct proportions, exert a beneficial 
influence on the activity of the organisms. 
The question of the best liquid nutrient medium having been thus far 
settled, the next point to be ascertained was whether the Bacteria are able 
to utilize the last traces of carbohydrate food material present in the culture 
solution, or whether, when the solution of the nutrient has become suffi- 
ciently dilute, the process of auto-digestion before mentioned sets in, 
no further increase in the nitrogen content taking place. It is generally 
recognized that the carbohydrate in 50 c c. of 1 per cent, solution is prac- 
tically used in seven days, so two series, each consisting of six flasks, were 
taken. These were numbered from 1 to 6 and from 7 to 12 inclusive. 
Each of the first six contained 50 c.c. of 1 per cent. Bottomley’s solution, 
and each of the second six was supplied with 50 c.c. of 1 per cent. Ashby’s 
solution, every one being provided with 50 grm. of sand to form a slope, 
and all being inoculated with 1 c.c. of the suspension of Azotobacter in 
sterile water. Nos. 1 and 7 were autoclaved at a temperature of 135° C., 
and the whole series was incubated at 28° C. The object of the experiment 
was to determine the fixation in each solution at the end of seven days, and 
also the fixation consequent on the consumption of the whole of the carbo- 
Since the completion of this paper, these results have been obtained, and are given on p. 885. 
