354 
ROBERT AARON STEINBERG 
Here we note that the total free acid in the culture solutions shows a 
regular and marked increase in total acidity only in the case of the zinc 
cultures. 
It is of course always to be remembered that in order to get an acidity 
sufficient to be of even a barely perceptible physiological significance as a 
"stimulant," there must be a concentration of the salt of the heavy metal 
which is very high from the standpoint of its direct physiologically "stimula- 
tive effect." This is of course a strong argument against the assumption 
that the effect produced by "stimulants" is due to the increased acidity of 
the solution resulting from their hydrolytic dissociation. Nevertheless, 
I am inclined, on the basis of the very positive evidence that an increase 
in the acidity of the Pfeffer solution results in a marked "stimulation" of 
growth, to ascribe an important physiological effect to the increased H-ion 
concentration resulting directly from the dissociation of these salts in the 
culture medium. It is perhaps not impossible that in case the "stimulants" 
are adsorbed we may get a membrane concentration which would bring the 
acidity more nearly within the range of strengths necessary to produce 
directly "stimulative effects" on growth. 
Thus it has been shown by Denham (13) that the hydrolysis of salts of 
this type can result in hydrogen-ion concentrations of considerable magni- 
tude. It will suffice to take the values found in n/i6 solution. This 
concentration is of course much greater than that often employed in the 
cultures. 
Salt(n/i6) [H+] PnCcalc.) 
TI2SO4 0.609 X 10-2 2.215 
CrCls 0.234X10-2 2.631 
AI2CI6 0.152 X 10-2 2.818 
Al2(S04)3 0.145 X 10-2 2.839 
C6H5NH3HCI 0.114X10-2 2.944 
NiS04 0.440 X 10-4 4.357 
C0SO4 0.107 X 10-4 4.971 
NH4CI 0.427 X 10-6 6.370 
The number of salts experimented with in this connection is so few that 
it is unfortunately impossible to ascertain whether any relation exists 
between pn and the optima for, or the magnitude of the acceleration of, 
growth. Nevertheless, it is evident that their action does not depend 
entirely on the acidity attained through hydrolysis, since trivalent aluminum 
has comparatively little "stimulative effect" in comparison with bivalent 
cobalt and nickel. Further, the increased yield due to increased acidity 
is much less than that brought about by the addition of zinc and iron. 
The addition of the majority results, however, in growth accelerations of 
the same magnitude as that caused by increased acidity. This variation 
in effectiveness of the different "stimulants" from the point of view of 
increased acidity may be accounted for on the basis of impurities, valence, 
differences in ease of adsorption, degree of hydrolytic dissociation, or by 
