162 
$8 4 The mutual proportion of the different ions, which 
are present in the fluids and the use of the 
solutions in series. 
The elements of the solutions deserve a further discus- 
sion. In the preceding chapter we have seen, that small 
quantities acid or base and solutions of neutral salts in a 
somewhat greater concentration have an influence on the 
motility of the Alga. The electrolytes are divided into ions 
for a great part in weak watery solutions. It is a matter 
of course to suggest, that the influence, exercised by the 
electrolytes, is excercised by the presence of the ions. In 
every watery solution are, besides the ions of the dissolved 
substance, also hydrions and hydroxylions, since the water 
has always been for a small part dissociated into H-ions 
and OHions. In really neutral solutions the number of 
H-ions and the number of OH-ions is equally great. 
However small quantities of carbonic acid, which easily 
dissolves in water, are the cause, that it is very difficult 
to obtain that the number of H-ions and of OH-ions is 
equally great in a solution. Moreover the quantity of 
dissolved carbonic acid is sometimes greater and then 
again smaller. The result of this is that the water, with 
which we make our solutions does not contain a definite 
concentration of H-ions and OHions. 
It is generally said, that solutions which are made from 
neutral salts with distilled water, are neutral. However 
for these solutions the acidity varies, as the acidity of 
the distilled water varies. For the phenomenon studied 
by us these small changes of acidity are of great impor- 
tance. Therefore it is desirable with a systematic examina- 
tion of the influence of the electrolytes on the motility 
of Chlamydomonas, to make use of series of solutions, 
which not only vary as regards the concentration of the 
electrolyte, which we examine, but also as regards the 
