8o 
Colouring of the Shells of Birds' Eggs. [January, 
from the specftrum of the light reflected from the eggs 
themselves or transmitted through broken fragments ; but in 
order to study colouring-matters in a satisfactory manner, 
it is requisite to obtain them in solution, so that they may 
be more or less separated from one another, their spedtra 
seen to greater advantage, and the effect of various reagents 
determined. In the shells of eggs the coloured substances are 
so intimately associated with carbonate of lime that they 
cannot be dissolved out ; and even when this has been 
removed, they are often so firmly enclosed in other insoluble 
organic substances, that it is difficult or impossible to dis- 
solve them out completely. In the majority of cases it is 
best to remove the earthy carbonates by means of somewhat 
dilute hydrochloric acid, added gradually until no further 
effervescence takes place. The character of the residue 
varies much in different cases. Sometimes we obtain a 
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 ; collecting 
this insoluble portion on a filter, washing well with water, and 
removingany largeportionsof colourless membrane and of the 
filter to which no colouring matter is attached, the coloured resi- 
due should be freed from superfluous water, but not actually 
dried, and placed whilst 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. Some- 
times 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. 
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 what I have already been able to learn suffices 
