1875.] COLOURING OF BIRDS’ EGGS. 359 
if not identical, substance does really appear to be a normal consti- 
tuent of the shell of eggs having a peculiar brick-red colour. 
Tue vARIoUS CoLouRs OF EGGS THEMSELVES. 
Such, then, is a general account of those peculiarities of the colour- 
ing matters that have come under my notice, which suffice to distin- 
guish them from one another and from analogous substances met 
with elsewhere ; and I now proceed to a more detailed consideration 
of the eggs themselves. As an illustration of the method of study, 
suppose that we have taken portions of the brownish-red eggs of the 
common Grouse, of the pure brown eggs of the Nightingale, and of 
the pure blue of the common Thrush, separated trom the black 
spots, kept for examination by themselves. After having, in each 
case, dissolved out the carbonate of lime with dilute hydrochloric 
acid and having washed the residues with water, they should each be 
digested in cold neutral absolute alcohol. Scarcely any colour would 
be dissolved out in the case of the Grouse—but a fine blue in all the 
others, which, on further examination, would be found to be oocyan, 
with mere traces of other substances. After having dissolved out as 
much as possible, by means of fresh neutral alcohol, the residue 
should be digested in alcohol with a small quantity of hydrochloric 
acid. It would then be found that the Grouse-shell would give a 
rose-coloured solution, containing much of the acid modification of 
oorhodeine. The Nightingale would also give much oorhodeine, 
but the colour would be modified by the presence of occyan; the 
blue portion of the Thrush-egg would give a small quantity of a fine 
blue substance, showing the spectrum of banded oocyan, with little 
or no trace of oorhodeine, whereas the dark spots would be found 
to give a very considerable quantity of oorhodeine. We thus clearly 
see that the redder egg is mainly coloured with oorhodeine ; the 
blue egg with oocyan—the brown colour of the Nightingale being 
due to mixture of these two, and the black spots on the Thrush-egg 
to patches containing much oorhodeine. All the various interme- 
diate shades of colour, passing from red through brown to blue, 
whether they occur in the eggs of different species or in the more or 
less variable eggs of the same kind of bird, or in patches on the same 
egg, can thus be explained without any difficulty. 
In a similar manner the various shades of green, passing from the 
blue-green of such eggs as those of the common Hedge Sparrow to 
the fine malachite green of the fresh Emu, and to the very yellow- 
green seen on them in patches, are all due to a variable mixture of 
oocyan with yellow ooxanthine. 
As is, no doubt, well known, many green eggs turn blue on long 
keeping. In this manner the beautiful malachite green of fresh 
Emu-eggs passes into dark blue. This is easily explained by the 
fact that yellow ooxanthine is much more easily destroyed by oxidi- 
zation than oocyan. A portion of a green Emu egg exposed to 
strong light soon becomes much bluer, and so does a mixed solution 
of the two colouring-matters in alcohol, the yellow constituent being 
destroyed and the blue left. 
