1875. ] 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 occa- 
sionally 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 carefuily 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 quan- 
tity 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 exten- 
sive 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 indi- 
vidual 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. Zoou. Soc.—1875, No. XXIII. 23 
