218 Experiments on the Growth of Yeast. [Sess, 
istic property of living matter. Let us take a simple example of 
what is meant. Tight boots increase corns. The boot presses 
on the foot: Nature resents the injury, and directs a company 
of cells to place a patch of leather on the irritated part. Simi- 
larly, the yeast-cells are stimulated by the poison, which is 
sufficiently weak to irritate and not to poison them, and in- 
creased action goes on. 
Our results were read at too long intervals to show this; 
but one of the curves (Plate XXVI., at end of fourth day), the 
one of 1 in 50,000, has a curious fall after a prominence. 
The fall we take to be due to the exhaustion after the 
effort to get rid of the poison. It is interesting to note 
how unexpected results record themselves mechanically in a 
curve. 
Since Hugo Schultz a number of other observers have 
made analogous observations. LEffront in 1894 or later 
found picric acid, salicylic acid, and formaldehyde, besides — 
hydrofluoric acid, to have a stimulating action. Other observers 
found that carbon dioxide made the protoplasmic movements 
in Elodea more rapid. There are, lastly, a number of very — 
suggestive researches on the etherification of cells, such as 
Spirogyra or seeds. We need hardly mention the effect of 
tea or tobacco or alcohol in man. ; 
3. At present the most striking, the most inexplicable, re- — 
sult of our experiments is the almost insignificant effect of what 
we have called moderate quantities of arsenious oxide on the ~ 
vital activity of yeast—quantities of 1 in 20,000 or 30,000 or © 
40,000 of water (Plate XXV.) No doubt a probable answer — 
will occur to some. It is well known that yeast as used by 
brewers, in this country at all events, is a mixture of organ- 
isms. It is also known that some kinds of fermentation 
are produced by two organisms acting together—a symbiosis 
like a green alga and a fungus ina lichen. The ginger-beer 
plant is such a symbiotic union, and kephir is obtained from 
milk by the mutual action of a butyric acid bacterium and 
a yeast. The arsenious oxide may be more injurious to some 
organisms in common brewer’s yeast than it is to others; but — 
this is not the place to enter into such a discussion, even 
if we were qualified to do so. Judging from the present 
trend of speculation on these matters, it is very likely to 
