372 The influence of light and of copper on fermentation. 
from the fermenting vessels largely determined the course of the 
fermentations. For example, the results obtained under the 
influence of blue light prcve that the inhibitory effect of this 
light on the development of acidifying bacteria, was to decrease 
the amount of acid produced in the fermenting wort, and, therefore, 
to lessen the amount of copper dissolved. On the other hand, the 
fermentations under the influence of red light and in the dark 
gave better facilities for a more vigorous growth of these bacteria. 
The acid produced thereby, dissolved a little of the copper ; and 
its inhibitory effect on a normal growth of the yeast, and a regular 
fermentation, is evident from the varying numbers obtained in the 
different determinations. The larger amount of CuO obtained in 
the fermentation in the dark as compared with the others appears 
to be evidence in favour of this view. But contributory causes 
may be owing to the differences of temperature ; for the tempera- 
ture of the fermentations in the dark and the red were usually 
a little higher than in the blue, and the solvent action of the 
acid on the copper would be thereby increased. At the same 
time it is probable that, as the fermentations proceeded, the 
C0 2 produced from the break down of the sugar, would also 
dissolve a little of the copper, and this also should be added as 
a further influence. 
(3) Another fact which is indicated by these experiments 
is the influence of the original composition of the wort. In 
a good wort, the yeast appeared to resist the action of the 
copper better than in a poor wort ; for the differences in 
acidity, &c., are much less marked in the good worts than in 
the poor ones. 
In view of these varying results, it will be of importance to 
study more systematically the influence of different amounts of 
copper, and of other metals like silver, upon the growth of yeast 
and fermentation, a problem which has been kindly suggested by 
Dr Horace Brown, who called our attention to some experiments 
conducted at Rothampstead on the influence of small quantities of 
copper and silver on the growth of various plants. 
The above investigation was conducted in the University 
Chemical Laboratory. 
