370 
ROBERT AARON STEINBERG 
2. Increase in acidity of the nutrient solution supplements the action 
of heavy metals (zinc, iron) ; decrease in acidity detracts from the action 
of heavy metals. 
3. Salts of the heavy metals which act as "stimulants" hydrolyze as a 
rule in aqueous solution so as to result in an increased acidity, and this 
increase in acidity, though slight, owing to the low concentration in which 
the salts are used, may possibly be considered as rendered effective through 
membrane concentration. Sodium silicate is an exception to this rule of 
increase in acidity by "stimulants," and it was found in my experiments 
not to produce effects like those of zinc on the growth, sporulation, etc., 
of A. niger. 
4. The progressive increase in acidity of the culture consequent upon 
the addition of a "stimulant" is, at least in major part, a result of the activi- 
ties of the organism, and may in turn be concerned also in the acceleration 
of growth and in the retardation of spore formation. 
5. Autoclaving at 14^ pounds the Pfeffer nutrient solution with calcium 
carbonate leads apparently to the removal to a high degree of the remaining 
traces of iron and zinc, and probably of all heavy metals present. 
6. Evidence is given that in the Pfeffer nutrient solution thus treated, 
practically no growth takes place; that the addition of either iron or zinc 
alone causes only a very slight increase in growth, but that when zinc and 
iron are added together a marked increase in growth takes place. 
List of Papers Referred to in the Text. 
1. Aso, K. The chemical composition of the spores of Aspergillus Oryzae. Bull. Coll. 
Agr. Tokyo Imp. Univ. 4: 81. 1900. 
2. Ayers, S. H. Hydrogen ion concentrations in cultures of streptococci. Journ. Bact. 
i: 84. 1916. 
3. Benecke, W. Die zur Ernahrung der Schimmelpilze nothwendige Metalle. Jahrb. 
Wiss. Bot. 28: 493. 1895. 
4. Bertrand, G., and Javillier, M. Influence du manganese sur le developpement de 
V Aspergillus niger. Compt. Rend. 152: 225, 1911, 
5. . Influence combinee du zinc et du manganese sur le developpement de 
V Aspergillus niger. Compt. Rend. 152: 901. 1911. 
6. Braconnot, H. See Czapek, F. Biochemie der Pflanzen, 2te Aufl. i: 148. 
7. Brenchley, W. E. Inorganic plant poisons and stimulants. Cambridge, 1914. 
8. Butkewitsch, W. Unwandlung der Eiweissstoffe durch die niederen Pilze im Zusam- 
menhange mit einigen Bedingungen ihrer Entwickelung. Jahrb. Wiss. Bot, 38: 
147. 1902. 
9. Clark, J. F. Electrolytic dissociation and toxic effect. Journ. Phys. Chem. 3: 263. 
1899. 
10. Clark, W. M., and Lubs, H. A. The colorimetric determination of hydrogen ion con- 
centration and its application in bacteriology. Journ. Bact. 2: 191. 1917. 
11. Currie, J. N. The citric acid fermentation of Aspergillus niger. Journ, Biol. Chem. 
31: 15. 1917. 
12. Czapek, F. Biochemie der Pflanzen I. 2te Aufl. Jena, 1913. 
13. Denham, H. G. The electrometric determination of the hydrolysis of salts. Journ. 
Chem. Soc. 93: 41. 1908. 
