352 MR. H. C. SORBY ON THE [May 4, 
Hitherto I have been able to distinguish seven well-marked sub- 
stances. One of these is identical with a colouring-matter met with 
in nearly all groups of plants, from the lowest to the highest; but I 
have not yet been able to identify any of the rest with any found 
elsewhere. But, at the same time, I must admit that our knowledge 
of animal colouring-matters is far too limited to make such negative 
evidence of much value. All these seven coloured substances found in 
the shells of birds’ eggs are insoluble in water, but soluble in absolute 
alcohol, either when neutral or when a small amount of free acid is 
present. They are also sometimes soluble in chloroform or carbon 
bisulphide. Absolute alcohol, however, is in every respect the most 
convenient and best solvent. Some are extremely permanent, and 
resist the action of powerful reagents, whereas others are of such 
unstable character that they are not only rapidly changed by acids 
or oxidizing reagents, but are even partially decomposed by evapo- 
rating their solutions to dryness at a gentle heat. In these general 
peculiarities they resemble bile-pigments more than any other group 
of colouring-matters, but do not actually agree with any that have 
passed under my notice. Some of them furnish us with a number 
of most interesting facts in illustration of the probable existence of 
a connexion between optical properties and chemical or molecular 
constitution; and the spectra of some of them throw much light on 
the theory of the arithmetical relations between the wave-lengths of 
the centres of absorption-bands, as I have shown in a paper read 
before the Royal Microscopical Society * ; but on the present occa- 
sion I forbear to enter into such questions, and will confine myself 
as much as possible to the zoological aspect of the subject. At the 
same time it is absolutely necessary to enter into a certain amount 
of chemical and optical details, since otherwise the characteristic 
peculiarities of the different substances could not be established. 
Meruop orf Stupy. 
In the first place, it may be well to remark that very little 
indeed can be deduced with certainty from mere general colour. 
Some important and reliable information may be learned from the 
spectrum of the light reflected from the eggs themselves or trans- 
mitted 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 spectra seen to greater advantage, and the eifect 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 sub- 
stances, that it is difficult or impossible to dissolve them out com- 
pletely. Inthe 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 
* Monthly Microscopical Journal, vol. xiii. p. 198. 
