430 NATURAL SCIENCE. Jone, 
fancied the poison might be a gas absorbed in the water and passing 
over in distillation. Carbonic acid, ammonia, and ozone were ruled 
out of the question, as any water in which Alge live necessarily con- 
tains these gases in greater or less quantity,and such water, from spring 
or river, pool or pond, was found to be harmless. Moreover, when 
these gases were added to the cultures no effect followed. There 
remained nitrous acid to be suspected, and the more so that Munich 
water was said to contain a good deal of this acid which would not 
be eliminated by distillation. A solution was made containing potas- 
sium nitrite and sulphuric acid, which would liberate free acid in the 
distilled water. The same results followed as before, namely, 
chemical poisoning in the stronger solution, and the oligodynamic 
effect when much diluted. Griess’s test was applied to estimate the 
quantity of nitrous acid in the ordinary water and in distilled water, 
but only once was there any reaction, and that so slight that no 
possible harm could be done by the amount present. 
Very remarkable results in rendering water oligodynamic were 
obtained by treating it with various bodies considered practically 
insoluble in ordinary water. These were copper, ‘silver, lead, iron, 
tin, and quicksilver. A whole series of experiments was tried with 
well-cleaned coins. Thus glass vessels, containing 100 or 500 cubic 
centimetres of water, were arranged with one, two, four, and eight 
gold coins. In these glasses were placed equal quantities of Sprrvogyra 
filaments. The plants in the glasses with the largest number of coins 
died first, the others succumbed more slowly, while the same species 
of Spwogyra lived in a healthy condition for weeks in ordinary 
spring water. 
Again, the effect was tried with neutral water in metallic vessels 
of silver and platinum ; the Spzvogyra filaments died as before, and if 
copper coins were dropped in a glass containing the filaments the 
effect was so far localised that those nearest the coins were attacked 
first. 
Nigeli had thus found an easy method of rendering water oligo- 
dynamic. A step further, and he made the still more remarkable 
discovery that he could again render the solutions neutral by the 
addition of quite insoluble bodies, trying first sulphur, soot, or 
graphite, then manganese, starch, flour, cellulose (filter-paper, cotton, 
linen, or wood), silk, wool, stearin, paraffin, etc., and the more of 
these bodies he put in, the quicker did the water recover its neutrality. 
He found that Algze themselves would act in the same way if he put 
in plenty of them; they, too, had power to render the water neutral. 
This enabled him to account for much that had been hitherto inex- 
plicable in the control experiments, where no proportion had been 
observed between the relative quantities of Algze and water. This 
was put definitely to the proof, and it was found that the more water 
used, and the fewer the filaments, so much the quicker did oligo- 
dynamic reaction take place. Colloid substances, such as gum, 
