231 
a whole, but also with those of different kinds of plants or 
trees. So far I have not been able to ascertain whether there 
is any one particular artificial oxidizing process which will in 
each case give rise to the exact products naturally found in 
the leaves themselves, but there is sufficient similarity to 
show what is the true character of the change, and that it is 
independent of vitality or the action of light, and is a mere 
chemical change in dead matter. It therefore appears almost 
certain that the rich brown tints of autumn are mainly due 
to the oxidization of previously existing more or less pale 
yellow colours of the chrysotannin group, and this agrees 
with the fact that the leaves then contain no tannic acid, 
or very little compared with what is found in them when 
they are green or yellow. ‘The erythrophyll appears to have 
very little, if any, influence on these final tints, for it is de- 
colorised by further oxidization; but the continued presence 
of the dull olive-coloured modification’ of chlorophyll com- 
pletely prevents the production of any fine, clear, bright 
brown. Asa general rule, therefore, such tints are seen to the 
greatest advantage in the case of leaves which first turn yellow 
or scarlet; and a good clean background having been thus 
formed, the subsequent oxidization of substances belonging 
to the chrysotannin group gives rise to the redder or yellower 
browns seen in the foliage of different trees. This will be 
better understood by means of a few special examples. ‘The 
leaves of the common beech first change into a bright yellow. 
They then contain, besides xanthophyll, a pale yellow colour, 
soluble in water, which is closely allied to, if not absolutely 
the same as, the quinotannic acid found in the dry bark of 
commerce, but more pure in the fresh green leaves of the 
cinchona plant. This kind of tannic acid when oxidized 
gives rise to several different coloured products, but, on the 
whole, they correspond very closely with the red- or brown- 
orange colour characteristic of the leaves at a later period. 
This colouring matter of the natural leaves is insoluble in 
water or neutral alcohol, but is easily dissolved in the latter, 
when made acid with hydrochloric acid; and then it agrees 
very closely with the product of the artificial oxidization of 
the pale yellow substance by means of nitrite of potash and 
hydrochloric acid, added to its solution in alcohol. The 
yellow leaves of the elm contain apparently the same tannic 
acid, along with a colour of the chrysophyll sub-group, 
which does not strike a dark colour with ferric salts. This 
when oxidized gives rise to a dark, dull brown substance. 
These two are mixed in varying proportions in the leaves of 
different elm trees, and thus some turn to a moderately bright 
VOL, XI.—NEW SER, Q 
