CHAP. II. FUNCTION OF NUTRITION. 195 



into different states of oxidation, each of which possesses 

 some peculiar tint, as in the case of the various oxides 

 of iron. Although carbon is the principle ingredient in 

 the composition of chrpmule, it is not likely as some 

 have supposed to be this substance in a perfectly pure 

 state. Although different colours in plants appear to 

 depend upon that action of light which effects the de- 

 composition of carbonic acid, yet we find that many 

 sea- weeds are intensely coloured when they grow at a 

 depth where the illuminating power of the sun's rays is 

 some hundreds of times less than it is at the surface of 

 the earth. Humboldt mentions having obtained the 

 Fucus vitifolius from a depth of 190 feet, where the 

 light which it received was two hundred and three 

 times less than that of a common candle placed at the 

 distance of one foot from the object illuminated. All 

 white flowers are only different tints extremely diluted 

 a fact of which the celebrated flower painter Redoute 

 availed himself. By placing the flower on a white 

 sheet of paper he could observe the exact tint, however 

 delicate, which ought to form the ground of his drawing. 

 All blacks on the other hand are only intense shades 

 of some of the darker colours, or of grey. 



(182.) Colours of Flowers. Colour is (generally 

 speaking) of very little importance as respects the de- 

 termination of species among flowering plants ; but it 

 often furnishes characters of considerable value for the 

 discrimination of many among the cryptogamic tribes. 

 In some other branches of natural history it is of much 

 greater consequence; and we shall here explain a method 

 by which an accurate and comprehensive nomenclature 

 may be established for defining colours, so far as may be 

 required in the description of objects of natural history. 

 The scheme is little more than a modification of a plan 

 suggested by M. Mirbel ; and consists in referring all 

 natural colours to certain absolute tints and shades*, 

 determined according to fixed rules. 



(183.) Composition of Colours. All colours may be 



* By " shade " we here mean the depth or intensity of a tint. 

 o 2 



