Aug. 19, 1918 
Reaction on Nitrogen-Assimilating Bacteria 
323 
The concentration of hydrogen ions was measured by the colorimetric 
method as outlined by Clark and Lubs (6). The procedure was as 
follows: To a 10-cc. portion of the culture fluid the proper indicator was 
added, and the color developed was compared with the colors obtained 
on the addition of the same indicator to tubes of 10 cc. of the various 
“buffer solutions” of known hydrogen-ion concentration. Since the 
accuracy of this method depends on the standard “buffer solutions,” 
these were prepared from chemicals purified as directed by Clark and 
Tubs, and the buffer mixtures checked by the electrometric method. 
In every case the two methods of measuring the hydrogen-ion exponent 
gave almost identical results. All of the data are reported as the hydro¬ 
gen-ion exponent or P H , instead of in terms of the normality of hydrogen 
ions. In the alkaline range, especially where large amounts of the base 
were used, the concentration of hydroxyl ions was frequently beyond the 
range of the indicators. Therefore the exponent of the hydrogen ion 
in the presence of large amounts of alkali is not correct. 
INFLUENCE OF SULPHURIC ACID AND SODIUM HYDROXID ON THE 
REPRODUCTION OF NITROGEN-ASSIMILATING BACTERIA 
TOTAL ACID AND ALKALI 
A considerable number of experiments were made with Rhizobium 
leguminosarum from different plants and in general the agreement 
between these tests was good; therefore only a few of the typical 
ones are presented. The data show that the legume bacteria vary 
in their resistance to acidity, depending on the source of the organism. 
Taylor (23) has shown very clearly that acids, especially organic acids, 
vary in their degree of activity in checking the growth of bacteria, 
but that the inorganic acids, hydrochloric and nitric, however, show 
much similarity of action. In this work no attempt was made to try 
out different acids or alkalis, but rather to measure the action of sul¬ 
phuric acid and of sodium hydroxid. 
EXPERIMENTS WITH ALFALFA BACTERIA 
In the following experiments a i-cc. suspension of legume bacteria in 
sterilized water was used to inoculate 100 cc. of mannitol medium (p. 321) 
in 750-cc. Erienmeyer flasks. At regular intervals of one week each the 
cultures were shaken vigorously and 1 cc. removed for plate counts. It 
was noticed early that increasing the acidity of a medium had a decided 
effect on the growth of the bacteria. Thus, the least acid members of a 
series were the first to show turbidity, while the more acid the reaction^ 
the longer the period required for a noticeable turbidity to appear. The 
results obtained are given in Table II. In mannitol culture medium 
the injurious effect of alkali on legume bacteria is not noticeable unless 
added in amounts greater than iV/125, while all growth is prevented in 
