Proceedings of the Scientific Meetings of the Zoological Society of London 1875

Page 423

1875.] COLOURING O F BIRDS' E G G S . 359 if not identical, substance does really appear to be a normal consti­tuent of the shell of eggs having a peculiar brick-red colour. THE VARIOUS COLOURS OF EGGS THEMSELVES. Such, then, is a general account of those peculiarities of the colour­ing matters that have come under m y notice, which suffice to distin­guish 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 from 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 iu 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 oocyan; 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. W e 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 interme­diate 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 oxidi­zation than oocyan. A portion of a green E m u 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.