170 
already, that the experiment goes smooth. If one has 
obtained a suitable series, then one must look for the 
acid solution, which yet permits to the algae to show 
the reaction to gravity, and the solution within which 
the reaction yet does not occur. The same thing is repeated 
with the alkaline solutions. Of these solutions the H-ion 
concentration is fixed electrometrically. For the acid 
solutions it was found, that with a hydrion concentration 
of 10—°°? the reaction just took place yet while with a 
concentration of 10 °° gramion hydrions per liter the 
reaction took no longer place. For the alkaline solutions 
these figures were 10-107 and 10-112. 
By this it is pointed out, that, at least partly, it depends 
on the H-ion concentration of a solution, whether or no 
the reaction to gravity is executed by the individuals of 
Chlamydomonas variabilis Dangeard. j 
À repetition of this experiment with mixtures of sodium 
malate and free malic acid or free sodium hydroxyde gave 
the following results. In the acid solutions of 0,01 molair 
sodium malate the limiting concentrations of H-ions were 
respectively 10-51 and 10-*°. For the alkaline solutions 
the figures were 10-106 and 10-11. It appears, that in 
the acid solutions of sodium malate the acidity was greater 
than in the solutions of sodium acetate. In the alkaline 
solutions the boundaries were not changed. We see from 
this, that besides the H- and OH-ions, also other ions 
have an influence on the reaction; even in the slight con- 
centration of 0,01 molair. It is of importance that the 
limiting concentrations in the alkaline solutions are not 
(at any rate very little) changed. This will be discussed 
further on. 
$ 2. Measures, to obtain reliable results. 
Now that it has become clear that H-ions, OH-ions, 
