148 
at least, not go smooth. Meanwhile it was of no use at 
all. The only thing we could think of then, was the increase 
of carbonic acid in the fluid, in consequence of the 
respiration, while assimilation of CO, was excluded. So 
ÏJ added to the water small quantities of aerated water, 
from which the excess of CO, had disappeared. It was 
proved that the quantity of carbonic acid could cause the 
algae to move downwards no longer. (See fig. 1D, pag. 34). 
We might suppose perhaps, that the salts from the aerated 
water, might be the cause of this phenomenon (aerated 
water is prepared from tapwater). This cannot be the 
case, however, since the algae in tapwater behave in 
about the same way as in distilled water; at any rate 
they give there a splendid show of the moving downwards. 
Further on I saw, that all sorts of acids are capable of 
annulling the positive geotaxis, moreover diluted bases 
and all kinds of salts which were active however in a 
somewhat higher concentration. 
With the microscope can be observed, that the motility 
of the algae has greatly diminished, when they move 
downwards any longer. 
[ got the impression that the change motile—immotile 
does not crop up all of a sudden, that on the contrary 
the decrease of the motility gradually changes with the 
increase of the concentration. Meanwbhile this cannot be 
stated with certainty, since complications act, which will 
be discussed more in detail further on. (See Chapter IV, 
$ 7). When examining with the microscope transition states 
in a weak white or red light, then the direction of the 
movements for the various individuals of a preparation 
is different. The number of those that swim upwards is 
probably rather great. There often takes place an accu- 
mulation in the upper layer of the fluid. Whether this is 
perhaps an indication of negative geotaxis, [ dare not 
decide. It is remarkable, that with phototaxis, similar 
