371 
of light and of copper on fermentation. 
D. 
From a consideration of the results of the fermentations in 
sections A., B. and C. it seemed to be desirable to conduct 
another series of fermentations in the four copper vessels, in 
order to discover what amount of copper, if any, was dissolved 
during the process of the fermentation. And, for this purpose, 
a litre of hopped wort was placed in each of the vessels, and 
the fermentations went on for five days under exactly the same 
conditions as those described in the preceding experiments. At 
the end of this time, the liquors were filtered, evaporated to 
dryness, and the residues ignited until all the organic substances 
were burnt away. What remained was dissolved in acid, and the 
CuO estimated in the usual way. Owing to an accident, the 
residue of the fermentation in the red light was lost ; but the 
following numbers show the amounts of CuO obtained from the 
three others, in percentages of the original wort : 
dark = 0-0072 °/ o CuO 
white =0-0051 ,, 
blue = 0-0052 
There can be little doubt, therefore, that the observed differences 
in the numbers obtained when the fermentations went on in the 
copper vessels were due to the solution of a little of the copper 
influencing the action of the yeast. And it is of interest to note 
the smaller quantity of CuO found in the fermentations in the 
white and blue than in the dark. 
General Results. 
The more important results of the preceding experiments are: 
(1) That fermentation when carried on under sterilised and 
unsterilised conditions in glass vessels, and under the influence 
of various spectral colours as well as of white light and also in the 
dark, was not seriously influenced, as shown by determinations 
of the optical activities of the fermented solutions. The small 
differences in the numbers and particularly of the acidities may 
be owing to experimental errors. Further experiments will be 
necessary to confirm these differences, and with a stronger light, 
like that of the arc. 
(2) That fermentation when conducted in copper vessels, but 
otherwise under the same conditions, is influenced very considerably, 
as shown by the marked differences in the numbers obtained from 
the acidity, optical activity, &c. It is not suggested that this 
influence was caused by rays of light of varying degrees of re- 
frangibility, except indirectly. It appears that copper dissolved 
