898 COLOURING MATTERS 



compounds. It is well known that those bodies when exposed to the rays of the sun, 

 especially when deposited in thin layers on or in fabrics made of animal or vegetable 

 materials, lose mueh of the intensity of their colour, and sometimes even disappear 

 entirely, that is, they are converted into colourless bodies. But whether this process 

 depends on a physical action induced by the light, or whether, as is more probable, it 

 consists in promoting the decomposing action of oxygen and moisture on them, is 

 uncertain. The most stable colouring matters, such as indigo-blue and alizarine in 

 its compounds, are not insensible to the action of light. Others, such as carthamine 

 from safflower, disappear rapidly when exposed to its influence. Colours produced by 

 a mixture of two colouring matters are often found to resist the action of light better 

 than those obtained from one alone. In one case, viz., that of Tyrian purple, tho action 

 of light seems to be absolutely essential to the formation of the colouring matter. The 

 leaves of plants also remain colourless if the plants are grown in darkness, though in 

 this case the formation of the green colouring matter is probably not due to the direct 

 chemical action of the light. 



The action of heat on colouring matters varies very much according to the nature 

 of the latter and the method of applying the heat. A moderate degree of heat often 

 changes the hue of a colouring matter and its compounds, the original colour being 

 restored on cooling, an effect which is probably due to physical causes. Sometimes 

 this effect is, without doubt, owing to the loss of water. Alizarine, for instance, crys- 

 tallised from alcohol, when heated to 212 F. loses its water of crystallisation, its colour 

 changing at the same time from reddish-yellow to red. At a still higher temperature 

 most colouring matters are entirely decomposed, the products of decomposition being 

 those usually afforded by organic matters, such as water, carbonic acid, carburetted 

 hydrogen, empyreumatic oils, and, if the substance contains nitrogen, ammonia, or 

 organic bases such as aniline. A few colouring matters, as for example alizarine, 

 rubiacine, indigo-blue, and indigo-red, if carefully heated, may be volatilised without 

 change, and yield beautiful crystallised sublimates, though a portion of the substance 

 is sometimes decomposed, giving carbon and empyreumatic products. 



Colouring matters, like most other organic substances, undergo decomposition with 

 more or less facility when exposed to the action of oxygen ; and the process may, indeed 

 be more easily traced, in their case, as it is always accompanied by a change of hue. 

 Its effects may be daily observed in the colours of natural objects belonging to the 

 organic world. Flowers, in many cases, lose a portion of their colour before fading. 

 The leaves of plants, before they fall, lose their green colour and become red or yellow. 

 The colour of venous blood changes, when exposed to the air, from dark red to light 

 red. When exposed to the action of oxygen, blue and red colouring matters generally 

 become yellow or brown ; but the process seldom ends here : it continues until the colour 

 is quite destroyed ; that is, until the substance is converted into a colourless compound. 

 This may be easily seen when a fabric, dyed of some fugitive colour, is exposed to the 

 air. The intensity of the colour diminishes, in the first instance ; it then changes in 

 hue, and, finally, disappears entirely. Indeed, the whole process of bleaching in the 

 air depends on the concurrent action of oxygen, light, and moisture. The precise 

 nature of the chemical changes which colouring matters undergo during this process 

 of oxidation is unknown. No doubt, it consists, generally speaking, in the removal 

 of a portion of their carbon and hydrogen, in the shape of carbonic acid and water, 

 and the conversion of the chief mass of the substance into a more stable compound, 

 capable of resisting the further action of oxygen. But this statement conveys very 

 little information to the chemist, "who, in order to ascertain the nature of a process of 

 decomposition, requires to know exactly all its products, and to compare their composi- 

 tion with that of the substances from which they are derived. The indeterminate 

 and uninteresting nature of the bodies into which most colouring matters are converted 

 by oxidation has probably deterred chemists from their examination. The action of 

 oxygen on colouring matters varies according to their nature and the manner in 

 which the oxygen is applied, and it is the degree of resistance which they are capable 

 of opposing to its action that chiefly determines the stability of the colours produced by 

 their means in the arts. Indigo-blue shows no tendency to be decomposed by gaseous 

 oxygen at ordinary temperatures ; it is only when the latter is presented in a con- 

 centrated form, as in nitric or chromic acid, or in a nascent state, as in a solution of 

 ferridcyanide of potassium containing caustic potash, that it undergoes decomposition. 

 "When, however, indigo-blue enters into combination with sulphuric acid, it is decom- 

 posed by means of oxygen with as much facility as some of the least stable of this 

 class of bodies. Some colouring matters are capable of resisting the action of oxygen 

 even in its most concentrated form. Of this kind are rubianine and rubiacine, which, 

 when treated with boiling nitric acid, merely dissolve in the liquid and crystallise out 

 again, when the latter is allowed to cool. The action of atmospheric oxygen on 

 colouring matters is generally promoted by alkalis, and retarded in the presence of 



