366 ROBERT AARON STEINBERG 
O.I Mg. Zn/L 
J? Ci KJ4J 1^ icloK. 
Yield 
Ph 
Sporulation 
0 mg. 
0.076 g. 
Fair 
I 
0.660 
1-2 
Excellent 
5 
0.822 
1-2 
Good 
20 
0.772 
1-2 
Good 
50 
0.840 
1-2 
Excellent 
100 
0.664 
1-2 
200 
0.591 
1-2 
Fe3(P04)2/Flask 
Yield 
Ph 
Sporulation 
0 mg. 
0.070 g. 
3-4 
Fair 
I 
0.553 
2-3 
Excellent 
5 
0.665 
1-2 
20 
0.784 
1-2 
50 
1.006 
1-2 
100 
1. 158 
1-2 
200 
1. 012 
1-2 
Mg. Zn/L 
Mg. Zn/L 
Yield 
Ph 
Sporulation 
0 
0.005 g- 
3-4 
Fair 
0.025 
0.047 
3-4 
< ( 
I 
0.079 
3-4 
(( 
5 
0.037 
3-4 
(( 
10 
0.030 
3-4 
Practically sterile 
25 
0.040 
3-4 
Practically sterile 
50 
0.037 
3-4 
Practically sterile 
it a 
100 
0.072 
3-4 
150 
0.040 
3-4 
200 
0.062 
3-4 
<( (( 
My experiments with the treated Pfeffer nutrient solution indicate that 
iron and zinc are in a different class as to efficiency from any of the other 
"stimulative" elements (cobalt, nickel, etc.) so far tested. It seems prob- 
able from these experiments, further, that the large growths obtained 
apparently with the use of zinc alone in my earlier experiments (64, 65) were 
in reality due to the combined action of zinc and iron, the iron being intro- 
duced as an impurity with the sugar or the other salts together with that 
intentionally added (45). The very limited growth obtained in the treated 
solution when either is used alone indicates that only by their combined 
action could the very marked increase of growth obtained in the experiments 
referred to have been induced. A further study of the composition of the 
treated solution is necessary, but there can be no question that it still 
contains as noted the necessary salts and sugar. Experiment 35, and 
experiment 33 as well, show that neither the calcium ion nor any compounds 
which it may form has any particular effect on the growth. The evidence 
seems conclusive that the treatment really results in a more thorough 
removal from the medium of all salts of zinc and iron than had hitherto 
been achieved in such experiments on growth "stimulation." While the 
prolonged exposure to high temperatures (about 150° C.) of such a mixture 
as the Pfeffer solution with its sugar and salts and the CaCOs may lead to 
