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Show 1 875-j COLOURING OF BIRDS' EGGS. 353 coloured membrane, occasionally like dark morocco leather, whilst in other cases the membranous part is very pale, and the colour chiefly occurs in detached skin-like flocks, or as minute particles disseminated through the liquid. By far the greater portion of these can easily be removed by filtration; but still in very many cases an appreciable quantity of the colouring-matter remains in the • liquid, in such a state of unstable solution that nearly the whole is deposited in an insoluble form on evaporating at a gentle heat. In any case the insoluble coloured residue should be washed with water on a filter, abstracting from it any considerable portions of pale or colourless membrane ; and after removing the greater part of the adhering water and the portions of the filter to which no colouring-matter is attached, it should be placed whilst still moist in absolute alcohol. This usually dissolves out a considerable amount of the colour ; but some still remains insoluble. A portion of this is occasionally soluble in alcohol containing free acetic acid ; but very often much remains undissolved until the residue is treated with alcohol containing hydrochloric acid. Sometimes even this fails to remove all, even when heated for many hours. All these different solutions should be kept separate, since they usually differ most materially ; and in no case should a strong acid solvent be used unless found to be necessary, because several of the normal colouring-matters are rapidly decomposed by strong free acids. For this reason it is in some cases advisable to separate the carbonate of lime from the shell by means of acetic acid ; but then unfortunately the colouring-matters are much less readily dissolved out of the residue by alcohol. DESCRIPTION OF THE COLOURING-MATTERS. These general remarks will I trust suffice to indicate the character of the methods usually employed ; and I therefore now proceed to describe the different coloured substances hitherto met with. The number of species of eggs which I have been able to carefully study is less than I could have wished; but at the same time, in looking over large collections, I have been able to see that those which I have studied in detail represent nearly the whole of the characteristic differences. What more especially remains to be done is to find examples of eggs which will enable us to prepare in a more pure state certain colouring-matters which usually occur in small quantity so mixed with others that some of their characters cannot be determined, to examine a few special eggs to ascertain whether they contain any substance not yet recognized, to compare a more extensive series of eggs in order to learn whether particular coloured substances are or are not characteristic of particular groups of birds, and to establish more completely the connexion between the individual colouring-matters of eggs and those found elsewhere. Such an inquiry would necessarily occupy much time ; and our knowledge of some important questions connected with the subject is very imperfect. However, what I have already been able to learn will, I trust, be sufficient to show how the methods I have employed will PROC. ZOOL. Soc-1875, No. XXIII. 23 |
