OF COLOURING MATTERS 36. THE knowledge of the laws according to which bodies appear of various colours, by absorbing some of the rays of light, and reflecting others, is of very little use to the practical dyer. His object is merely to ex tract the colouring principle from the substances which form its original basis; to transfer it to the stuff; to modify it by those agents which have been found by pre vious experiment to affect its nature; and to give it per manency in its new state of combination. Still, however, it may not be altogether foreign to our purpose to take a slight view of the theoretical opinions which are enter tained on this subject.
37. The property which bodies possess of uniformly reflecting certain rays of light, constitutes their colour: when all the rays are absorbed, a body appears totally destitute of colour, or black ; and when particular rays are absorbed, and others reflected, it seems to be of the colour which the reflected ray is capable of exciting in our organs of vision. When the substances which thus appear coiourcd arc transferred to colourless bodies, they generally communicate to these bodies their particular colour; and when they are transferred to bodies already coloured, they blend or mingle their colour with that of the bodies to which they are applied, and give rise to mixed colours.
38. The aptitude of reflecting particular rays of light, seems to depend not so much upon the chemical consti tution of a body, as upon a certain arrangement, or dis position of the particles of the surface. Many bodies display different colours, according to the particular an gle under which they are viewed; while others assume a change of colour, simply by the change of their me chanical condition.
39. Hence it appears that colour ought not to be re garded as a distinct principle, existing separately from the coloured body ; but merely as a faculty which the constituent elements of bodies possess of reflecting par ticular rays of light decomposed at their surface, and which is capable of being modified by changes in the mechanical structure, or in the proportion of the ele mentary p.trts. It is therefore impossible to predict, a priori, the colour of a compound body, from a know ledge of the principles of which it is composed. Fre quently two colourless bodies form a coloured compound by mixture; and it often happens that two substances, each of which has a very deep colour, are rendered en tirely colourless when they are united together. In
short, Ave can only determine by observation and experi ment, the colours which shall result from particular combinations of bodies.
4.0. These views, however, do not altogether accord with the opinions of several eminent chemists, particu larly with respect to colours of vegetable origin. Many seem to think that a peculiar proximate principle exists in vegetables, in which their colour generally resides, and to which they have given the name of colouring matter. " In this opinion," as Mr Murray justly ob serves, ,4 there is a degree of obscurity and vagueness." Colour being, as we have already stated, a secondary quality, which may reside in any principle, and being often exhibited by principles of the most opposite kind, there is no good reason for supposing that there is any distinct principle to which it exclusively belongs. At the same time, it must be admitted that there is appa rently a similarity in chemical constitution between cer tain vegetable products, in which colouring matters re side in the deepest intensity; and that these colouring matters may be detached from their state of combination by the agency of various solvents, and transferred to other substances with which they appear to have a more powerful affinity, and this too without our being able to refer the appearances which take place to any known proximate principle.
41. These facts, however, by no means authorise us to conclude that the colouring matter is a separate and independent principle ; all that can be inferred from them is, that the colouring matter does not uniformly exist in combination with any particular proximate principle, but probably with different proximate principles, or various combinations of them ; though the present state of che mical analysis does not enable us to ascertain the compo sition of the colouring particles with a degree of preci sion sufficient to detect the principles to which they owe their properties. Without taking any notice, therefore, of the theories which have been advanced to explain the cause of colour, we shall be satisfied with presenting a general view of the different colouring matters, and leave to future investigation the task of discovering whe ther these colouring matters form separate substances, or exist uniformly in combination with some particular proximate principle as a basis.