TTiOKAL COLOURS AND PIGMENTS. 



465 



its freedom from admixture with white light. Thus a small amount of 

 violet light mixed with a large amount of white light will barely tint the 

 latter, whilst unmixed the colour is pure or very decided in tint, though 

 of slight luminosity. By the admixture of white light we get chalky 

 blue and reds &c. 



The colour of a solid pigment is not always the same as that of its 

 solution, and the tone of the solution may vary according to its strength 

 and the presence or absence of other even uncoloured substances in 

 solution with it. Thus there are plant pigments which are yellow in 

 weak solution, orange in stronger solution, and red in still stronger 

 solution. There are also plant pigments which are red in a solution with 

 free acid, and blue in a solution without acid or with an alkali. The 

 solvent also has often an effect on the colour of the solution. These 

 phenomena are connected with the modern physico-chemical theory of the 

 electrolytic dissociation of molecules in solution. The theory is that the 

 molecules of many substances, when they enter into dilute solution, are 

 decomposed into positive and negative ions, and that the light absorption 

 of such a solution, and therefore its colour effect, is made up of the 

 combined absorptions of the ions. The solid is wholly made up of un- 

 dissociated molecules, whilst in a, stronger solution dissociated and 

 undissociated molecules are present together. ^Yhen the undissociated 

 molecule is coloured the tone of the ion is often different. It therefore 

 follows that the hue of the solid is sometimes different from that of a 

 strong solution which contains both ions and complete molecules, and 

 the hue of a weak solution which may contain none but completely 

 dissociated molecules is again different. The addition of various sub- 

 stances to the solution, especially of acids or alkalis, may cause the 

 dissociation to advance or retreat, and so alter the colour. 



A good example is to be found in the well-known substance, litmus, 

 which is a plant product used as an indicator in chemistry. In this, both 

 the undissociated molecule and the ion are coloured, but in different ways. 

 The solid neutral substance is blue. In stronger aqueous solution it is 

 also blue. If an acid be added to the blue solution the dissociation of the 

 molecules is increased, and the free ions thus produced are red. If an 

 alkali be now added to neutralise the acid the dissociation retreats and 

 the solution is again blue. The presence of acid, alkaline, and other 

 substances in solution in the cell-sap of plants, together with pigments, 

 has a great influence on the colours of the pigments. 



Flowers are viewed by the light which is given off from their surfaces, 

 much of it after having penetrated to a greater or less extent, and not by 

 that which is transmitted through their tissues. In a white flower the 

 cell-walls and the cell-contents are all colourless and transparent. Of the 

 white light incident upon such a flower, a portion is reflected, from the 

 outer walls of the epidermis. If these walls be at all papillate and 

 striated, as is usually the case, then this portion is scattered in all direc- 

 tions, or is irregularly reflected, as it is called ; if they be smooth, this 

 portion of the incident light is regularly reflected and the surface is 

 polished or shiny, as is the case with the floral segment of a Buttercup, 

 the skin of a Bean, and the surfaces of many leaves. The rest of the 

 incident light enters the epidermal cells, passes through their contents — 



