[a ey a 
IN MIxeD SOLUTIONS 397 
to the potassium salt is added, the growth-rate is still further in- 
creased. In the last mixture we have five salts, each, with the ex- 
ception of the calcium compound, in a concentration strong enough 
to interfere distinctly with growth. As a result of their presence 
together, not only is there no addition of poisonous effects, but a 
neutralization of toxicity to such degree as to permit in the mixed 
solution a growth-rate equal to or greater than that seen in the 
check culture. 
When the concentration of copper solution was doubled and 
the concentration of the other salts left as before, we found that 
the action of the copper was more slowly overcome, and even in 
the most complex mixtures studied, the growth-rate was still be- 
' low that of the check. Apparently, the poisonous activity of the 
copper in these casés was greater than such as could be neutralized 
by the quantities of other salts added to it. When, on the other 
hand, the concentration of the copper solution was kept as in the 
first instance and the concentration of the lighter salts added was 
diminished by half, the neutralizing action of the latter was mark- 
edly less. In the most complex mixtures under these latter con- 
ditions the observed growth-rate only equaled that of the control. 
Apparently this fact was due to the unneutralized copper action, 
since each of the other salts present were below a harmful con- 
centration. 
Returning, now, to mixtures in which the anion of the copper 
salt is not duplicated in any of the other salts present, we see a 
result essentially like that just noted. When to copper acetate, 
for example, salts of the metals used before are added in quantities 
equal to those indicated in Table VI, a similar result is seen. 
The growth-rate in the pure copper salt in this case is somewhat 
greater, since the CH,.CO, anion is slightly less poisonous than the 
Cl or the SO, anion. The addition of the sodium salt again in- 
creases the toxicity of the mixture. The further addition of the 
magnesium salt diminishes the harmful action somewhat, the activ- 
ity of the mixture being, roughly, the same as that of copper ace- 
tate alone. The entrance of the calcium salt, as before, produces 
a marked acceleration of growth, the rate jumping to a point con- 
siderably above the control. The final addition of the potassium 
salt still further increases this stimulation. As a result of this ex- 
sii 
