Hoyt: CULTURES OF SPIROGYRA 339 
the experiments,—by determining its effect, when used alone, 
upon Spirogyra filaments. Out of more than ten runs the distillate 
from but a single one was found to be decidedly toxic to the alga. 
The cause of this single failure was not investigated. The new 
method was uniformly employed for the preparation of nontoxic 
water throughout the course of these studies. 
Several experiments were carried out to throw light upon the 
question of the manner of operation by which distillation from 
animal charcoal may produce such a marked effect upon the physio- 
logical properties of the Heidelberg water. Not only the distillate 
but also the undistilled and concentrated residue (after removal 
of the charcoal by filtration through paper) showed marked 
improvement as compared with the untreated tap water (II, 11, 
12). Furthermore, a high degree of toxicity was produced in the 
practically nontoxic water, either of the filtered residue or of 
distillate, by the addition of a portion of the used charcoal that 
had been retained in the filter (II, 11, 12a, 12c). 
From the above observations it appears clear that the animal 
charcoal in the boiler of the still must have acted to remove toxic 
substances from the water, although, as has been stated, such 
action could not be detected at ordinary temperatures. It also 
appears that charcoal which has been the agent in this removal of 
injurious material becomes itself able to reproduce toxic properties 
in nontoxic water to which it may be added. It may be therefore 
considered as at least highly probable that the action of the finely 
divided carbon of these distillation experiments was that of an 
absorber rather than that of an oxidizing or other catalytic agent. 
Of course it is open to question, whether the toxicity reproduced 
in harmless water by used animal charcoal may be caused by the 
same substances as was the original toxicity of the tap water before 
treatment. The state of our knowledge of these matters precludes 
their further analysis at this time. 
One experiment was made to show the relative resistance to 
the toxic influence in tap water of two portions of Spirogyra which 
were originally from the same source but which had been subjected 
for a time to different conditions. One sample was taken from 
the stock jar and another from a vigorous culture which was origi- 
nally from the stock culture and then had grown for the last 68 
