3G1 
colours are amongst the most difficult to distinguish from 
one another, and I have not yet been able to discover any 
more satisfactory method than the following : — A slight 
excess of ammonia should be added to the alcoholic solution ; 
made so dilute that the absorption in the orange part of the 
spectrum is distinct, but yet very far from black, and the 
position of the limit of the absorption towards the red end 
carefully measured and recorded, thus — If .... 
The same limit should also be ascertained after consider- 
able excess of hydrochloric acid has been added to the 
solution, both in water and in alcohol ; which solutions 
sometimes give the same, but sometimes materially different, 
spectra. These various limits of absorption vary with the 
depth of colour, but by taking care to have solutions which give 
equally strong absorptions, and comparing them side by side, 
it is often very easy to distinguish from one another very 
closely allied colours — for example, that of dark gooseberries 
from that of grapes, or that of fresh grapes from that of new 
dark wines. The difference between these latter is proved, 
not only by the spectra, but also by other facts. 
In my paper in the ‘ Proceedings of the Royal Society,’ 
already cited (p. 443), I showed that when certain colouring 
matters are dissolved in water or alcohol, they give deep- 
coloured solutions, which rapidly fade to nearly colourless ; 
but when these faded solutions are evaporated to dryness, 
the colour is completely restored to its original state, and is 
also made quite as dark as if it had never faded, by adding 
some strong acid ; so that the fading is not due to decom- 
position, but to a molecular change rapidly taking place in 
dilute solutions. The colour of fresh grapes is an example 
of this ; and by comparing with a standard which did not 
fade, the solution immediately after the colour was dissolved, 
and the same after less than an hour, when no further change 
occurred by keeping it longer, I found that the faded re- 
quired fully five times the thickness to give the same intensity 
of absorption at the yellow end of the green. On the con- 
trary, new dark wines evaporated to dryness do not fade at 
all in such a manner when redissolved in water. 
This colour of new wines appears to be produced at a very 
early part of their formation. By dissolving the colour of the 
fresh grapes in the juice, adding a little yeast, and keeping 
the solution warm for a few days, the colour appeared to be 
changed into that found in new wines, and did not fade at 
all when redissolved in water. The colour kept as dry syrup 
for three years showed a similar change. I think it must be, 
therefore, due either to fermentation or to a slight oxidiza- 
