896 



COLOURING MATTERS 



COLOURING MATTERS. The colour of any object, either natural or artificial, 

 owes its origin to the effect produced on it by the rays of light. This effect is either due 

 to the mass or substance of the body itself, as may be seen in the colours of metals and 

 many shells, or it arises from the presence of some foreign substance or substances 

 not absolutely essential to it, and which may in many cases be separated and removed 

 from it. It is in speaking of these foreign substances which are often found colouring 

 natural objects, or which are employed in the arts for the purpose of imparting colours 

 to various materials, that we generally make use of the term COLOURING MATTER. 

 By chemists, however, the term is only applied to organic bodies, and not to mineral 

 substances, such as oxide of iron, cinnabar, ultramarine, &c., which, though they are 

 employed as pigments in the arts, differ very widely in their properties from one 

 another and from colouring matters in the narrower sense of the word. Colouring 

 matters may be defined to be substances produced in animal or vegetable organisms, 

 or easily formed there by processes occurring in nature (such as oxidation or fermen- 

 tation), and which are either themselves coloured or give coloured compounds with 

 bases or with animal or vegetable fibre. According to this definition, bodies like 

 carbazotic acid and murexide, which are formed by complicated processes such as 

 never occur in nature, are excluded, though they resemble true colouring matters in 

 many of their properties, such as that of giving intensely-coloured compound 

 bases. Whether, however, even after accepting the above definition, colouring 

 matters can be considered as constituting a natural class of organic bodies, such as 

 the fats, resins, &c., must still remain doubtful, though modern research tends to prove 

 that these substances are related to one another by other properties besides the acci- 

 dental one of colour, and will probably be found eventually to belong in reality to one 

 natural class. 



