Hoyt: CULTURES OF SPIROGYRA 343 
experiments appear to corroborate each other, and seem to throw 
some light on the general nature of the toxic substances present in 
these waters. It will be noticed that'the effect'of distillation and of 
high temperature was often different in the case of tap water and 
in that of ordinary distilled water. Some of these differences 
are summarized in TABLE Iv, where a plus sign indicates improve- 
ment, a minus sign indicates no improvement. When both signs 
occur, different tests were in disagreement. 
The facts of TABLE Iv indicate that the toxic materials present 
in the tap water and in the ordinary distilled water here studied 
were, in part at least, different substances; whatever may have 
been the nature of these substances those in the ordinary distilled 
water were mainly very resistant to heat. 
As to the general characteristics of the toxic substances in 
the ordinary distilled water, there appears no reason for doubting 
that they were probably, for the most part, metallic in their 
nature, derived in some way from the distillation apparatus. This 
is the conclusion reached by many authors who have studied the 
toxicity of ordinary distilled waters since the time of Nageli, and 
such supposition introduces no difficulty in the interpretation of 
the experiments that are reported here; it might be expected that 
metallic poisons, whether dissolved as salts or ions or existing 
merely in suspension, would not be very sensitive to correction by 
heat. The effect of redistillation might be to transfer a portion 
of the poisons from retort to receiver, thus leaving the distillate 
more or less toxic, and at the same time concentrating another 
portion (nonvolatile with steam and not sensitive to the tempera- 
tures employed) in the retort. 
It has been shown by Livingston et al. (11), Livingston (12), 
Wheeler and Breazeale (37), Schreiner and Reed (27, 28, 29), 
Shorey (30, 31), and Lathrop (26), that certain organic substances 
Present in various soil extracts are toxic to wheat seedlings. 
Since, as was pointed out by Livingston (12), tap water may be 
considered as a natural soil extract, the possibility is suggested 
that it may be toxic on account of poisons derived from the soil. 
At the same time, it is of course probable that the toxicity of the 
Heidelberg tap water here tested may have been due, to a greater 
or less extent, to substances having their origin in the supply pipes. 
