at the Roijal Institution, 1 900-1 907. 377 



of the refractive powers which such systems would exercise separately 

 but always greater. The system of carbon atoms forming benzene 

 also has a peculiar influence on light and when such systems are 

 combined the effect they exercise is in excess of the sum of their 

 influences apart. 



Colour may even arise by the superposition of the effects pro- 

 duced at ethenoid junctions and in benzenoid systems— in fact, it is 

 now beyond question that colour is conditioned by structure and it 

 is doubtful if any really simple molecule be coloured. Although 

 benzene and naphthalene are colourless, anthracene, as already men- 

 tioned, is coloured (pale yellowish-green) ; moreover it is fluorescent. 

 It will be noticed, on reference to the formula on p. 375 that anthracene 

 is represented as containing four ethenoid systems and a centric 

 benzenoid system : its colour is doubtless due to the co-operation 

 of these, each system acting as a special " light-absorbing " or 

 resonating centre but the effect produced in this case, judging from 

 the intensity of the colour, is not very considerable. Yellow and red 

 hydrocarbons are also known : thus the red colouring matter of the 

 carrot is a hydrocarbon ; there is reason to suppose that the number 

 of ethenoid and centric systems in these compounds co-operating in 

 the production of colour is larger than in anthracene. 



Eelatively few coloured hydrocarbons are known, however. The 

 majority of coloured organic substances are hydrocarbon derivatives 

 containing either oxygen or nitrogen or both these elements in 

 certain definite forms of combination. To give an example, 

 Faraday's benzene (CgHg), which although highly refractive, is 

 colourless, is easily converted into a coloured substance by means 

 of oxygen. By introducing in place of two of its hydrogen atoms 

 two hydroxyl groups — that is to say, the fundamental molecule of 

 water, OHo, minus one of the atoms of hydYogen^hj/droqidnone or 

 quinol is produced, the substance well known among photographers, 

 who use it as a developing agent. Quinol is a non-volatile, odour- 

 less, colourless crystalline substance which dissolves easily in water. 

 It is very sensitive to the action of oxidising agents, whereby it is at 

 once deprived of the two atoms of hydrogen contained in the two 

 hydroxyl groups and converted into quinone, which is so called as it 

 was first obtained by oxidising quinic acid from cinchona bark. The 

 change may be pictured in the manner shown on next page. 



Thus represented, the change involves a striking alteration in 

 the contexture of the molecule, in fact, the passage of the oxygen into a 

 special state such as Faraday contemplated. Corresponding to this 

 change, an extraordinary alteration in properties is noticeable, 

 quinone being a deep yellow coloured, highly volatile solid, of 

 marked odour, scarcely soluble in water. Its colour may be ascribed 

 to the presence in the one molecule of the two ethenoid (oxygen- 

 carbon) junctions and the benzenoid centric junction, which together 



