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Show 352 MR. H. C. SORBY ON THE [May 4, Hitherto I have been able to distinguish seven well-marked substances. 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 m u c h 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 evaporating 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 m y 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 occasion 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. METHOD OF STUDY. 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 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 spectra 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 dissolve 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 m u c h in different cases. Sometimes we obtain a * Monthly Microscopical Journal, vol. xiii. p. 198. |