CHEMICAL STIMULATION OF GROWTH OF ASl»ERGILLUS NIGER 345 
There appeared no difference with respect to sporulation between the controls and 
the sodium silicate cultures. The acidities of all cultures at the time of harvest corre- 
sponded to ph = 3-4. 
18. The effect of adding sodium silicate to the culture medium. 
Strain W, ii 
NazSiOa (Kahlb.) 
Yield 
NazSios (Kahlb.) 
Yield 
0.% 
0.176 g. 
0.12% 
0.236 g. 
0.002 
0.195 
0.18 
0.139 
o.oi 
0.154 
0.26 
0.126 
0.05 
0.230 
0.32 
0.144 
0.08 
0.207 
0.40 
0.162 
No differences in spore formation appeared. Acidities at the time of harvest were 
equivalent to pn =3 — 4. 
My cultures showed none of the ordinary "stimulation" effects, and 
I am inclined to associate these results with the fact that the addition of 
the silicate does not result in the increased acidity of the nutrient solution. 
Further exceptions to this general rule that ''stimulants" are electro- 
lytes whose cations form weakly ionized bases of low solubility (i.e., dis- 
sociate hydrolytically in the presence of an anion of a strong acid so as to 
cause an increase in acidity) are found possibly in experiments by Richards 
(57) and Yasuda (69) indicating that alkaloids, and by Miss Latham (28) 
indicating that chloroform, may cause growth "stimulation." 
Finally there remain the substances used by Schulz (62) in his studies 
on acceleration of respiration with yeast: HgCl2, I, KI, CrOs, sodium 
salicylate, formic acid, and arsenious acid; and those used by Fred (16) 
in his studies on bacteria: ether, carbon disulphide, potassium dichromate, 
CUSO4, and salvarsan. Of these thirteen substances, six agree in their 
ability to cause an increase in acidity; two must be grouped with chloro- 
form as comparatively chemically inert, and the position of the others: 
I (and KI), Br, sodium salicylate, and salvarsan are doubtful. 
It is to be noted for all these cases that practically the difficulty is not 
to induce the appearance of the phenomena associated with zinc "stimula- 
tion," but to prevent their occurrence. Substances exercising an action 
analogous to that of zinc may very easily be accidentally introduced through 
use of improper glassware (64) and impure chemicals. Particularly is one 
to exercise great caution in the interpretation of results obtained with 
compounds in high concentration, since any impurity they contain may 
thus be presented to the organism in effective concentration. An impurity 
of one-thousandth of one percent may cause the introduction in five percent 
solution of a compound of 0.5 mg./L impurity. 
Further I have no wish to suggest that the degrees of growth "stimula- 
tion" are always the result of varying acidities. Without doubt, it is 
possible that even if the majority of these substances actually function 
through their effect on the acidity of the nutrient solution, others by in- 
