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optimal acid-concentration for Chlamydomonas must lie 
near the turning over of methylred. This takes place at 
a hydrion concentration of about 10°. So this solution 
contains about 0,00001 gram-ion of hydrions. From all 
this it appears with what minimum concentrations we 
have to work. 
Î can make clear in another way, that for Chlamy- 
domonas the threshold value must be very small When 
I put the A/gae into a solution, the hydrion concentration 
of which was fixed in an electrometrical way, it: became 
clear to me, that with a hydrion concentration of 10->, 
the organism did hardly move, that they no longer reacted 
on the light, neither to gravity. During experiments with 
a capillary tube, filled with acid and in which one gets 
a ring-shaped accumilation, the algae at the inside of the 
ring, are inclined to be at rest. The greater part in the 
middle and at the outside of the ring does not show this 
inclination. The state of things, which I find with a H-ion 
concentration of 10°, probably lies above the optimum. 
How these experiments took place, will be proved further 
on. We arrive at the conclusion, that Chlamydomonas 
must be susceptible to acid concentrations smaller than 
100000 gram-ion of H-ions per liter. 
From the above-mentioned statements we can draw 
the conclusion that we must not expect from the capillary 
method: 
1°. that it can provide us with useful definitions of the 
threshold-values. 
2°. that with the qualitative examination as regards the 
chemotactical activity of solutions, a negative result actually 
denotes insensibility for the solution. 
If we use the capillary method for biological investi- 
gations, [ believe, that it is very suitable for those. Thus 
it is a matter of course that with biological questions (for 
instance with the fructification) so small a sensibility for 
