COLOURING MATTERS 897 



Colouring matters occur in all the organs of plants, in the root, wood, bark, leaves, 

 flowers and fruit ; in the skin, hair, feathers, blood, and various secretions of animals ; 

 in insects, for example, in various species of coccus; and in mollusca, such as the 

 murex. Indeed there are very few plants or animals whose organs do not produce 

 some kind of colouring matter. It is remarkable, however, that the colours which are 

 most frequently presented to our view, such as those of the leaves and flowers of 

 plants and the blood of animals, are produced by colouring matters with which we are 

 but very little acquainted, the colouring matters used in the arts, and which have been 

 examined with most care, being derived chiefly from less conspicuous organs, such as 

 the roots and stems of plants. In almost all cases the preparation of colouring 

 matters in a state of purity presents great difficulties, so that it may even be said that 

 very few are known in that state. 



Some colouring matters bear a great resemblance to the so-called extractive matters, 

 others to resins. Hence they have been divided into extractive and resinous colouring 

 matters. These resemblances are, however, of no great importance. The principal 

 colouring matters possess such peculiar properties that they must be considered as 

 bodies altogether sui generis. 



As regards their most prominent physical characteristic, colouring matters are 

 divided into three principal classes : viz., the red, yellow, and blue, the last class com- 

 prising the smallest number. Only one true green colouring matter occurs in nature, 

 viz., chlorophyll, the substance to which the green colour of leaves is owing. 1 Black 

 and brown colouring matters are also uncommon, the black and brown colours 

 obtained in the arts from animals or vegetables being (with the exception of sepia and 

 a few others) compounds of colouring matters with bases. The colours of natural 

 objects are often due to the presence of more than one colouring matter. This may 

 easily be seen in the petals of some flowers. If, for instance, the petals of the orange- 

 coloured variety of the Tropaolum majus be treated with boiling water, a colouring 

 matter is extracted which imparts to the water a purple colour. The petals now 

 appear yellow, and if they be treated with boiling spirits of wine, a yellow colouring 

 matter is extracted, and they then become white. When the purple colouring matter 

 is absent the flowers are yellow ; when, on the contrary, it is present in greater abun- 

 dance, they assume different shades of brown. Precisely the same phenomena are 

 observed in treating the petals of the brown Calceolaria successively with boiling 

 water and spirits of wine. In many cases colouring matters exhibit, when in an 

 uncombined state, an entirely different colour from what they do when they enter 

 into a state of combination. The colouring matter of litmus, for instance, is, when 

 uncombined, red, but its compounds with alkalis are blue. The alkaline compounds 

 of alizarine are of a rich violet colour, while the substance itself is reddish-yellow. 

 Many yellow colouring matters become brown by the action of alkali, and the blue 

 colouring matters of flowers generally turn green when exposed to the same influence. 

 The classification of colouring matters, according to colour, is therefore purely arti- 

 ficial. The terms red, yellow, and blue colouring matter, merely signify that the 

 substance itself possesses one of these colours, or that most of its compounds are re- 

 spectively red, yellow, or blue. In almost all cases, even when the colour is not entirely 

 changed by combination with other bodies, its intensity is much increased thereby, 

 substances of a pale yellow colour becoming of a deep yellow, and so on. 



Colouring matters consist, like most other organic substances, either of carbon, 

 hydrogen, and oxygen, or of those elements in addition to nitrogen. The exact relative 

 proportion of these constituents, however, is known in very few cases, and in still fewer 

 instances have the chemical formulae of the compounds been established with any ap- 

 proach to certainty. This proceeds on the one hand from the small quantities of these 

 substances usually present in the organs of plants and animals, and the difficulty of ob- 

 taining sufficient quantities for examination in a state of purity, and on the other hand 

 from the circumstance of their possessing a very complex chemical constitution and 

 high atomic weight. 



Only a small number of colouring matters are capable of assuming a crystalline 

 form ; the greater number, especially the so-called resinous ones, being perfectly amor- 

 phous. Among those Which have been obtained in a crystalline form, may be men- 

 tioned alizarine, indigo-blue, quercitrine, morine, luteoline, chrysophan, and rutine. It 

 is probable, however, that when improved methods have been discovered of preparing 

 colouring matters, and of separating them from the impurities with which they are so 

 often associated, many which are now supposed to bo amorphous will be found to be 

 capable of crystallising. 



Very little is known concerning the action of light on colouring matters and their 



1 Another green colouring matter, derived from different species of Rhamnut, has been described 

 under the name of ' Chinese Green.' It is stated to be a peculiar substance, not as might be sup- 

 posed, a mixture of a blue and a yellow colouring matter. 



VOL. I. 3 M 



