THE RELATIONSHIP BETWEEN THE 



COLOUR AND CONSTITUTION OF 



ORGANIC COMPOUNDS 



By S. J. MANSON AULD, Ph.D. 

 Scientific and Technical Department, Imperial Institute 



Simultaneous with the growth of Organic Chemistry, and the 

 consequent wide extension of synthetically formed bodies with 

 definite constitution, there has grown up the desire to draw 

 a direct connection between their physical properties and 

 chemical constitution. In no case has this been more exem- 

 plified than in the property of "colour," which from its very 

 nature is, as a rule, the first to become apparent to the senses. 

 Its investigation has been all the more attractive and has 

 received greater attention because of the necessity of 

 building up a certain " chemistry of colour " to aid in the 

 systematic synthesis of the organic dye-stuffs and colouring- 

 matters. The properties of colour and dyeing-power must, 

 however, be kept distinctly apart, and in discussing the former 

 there is no need to inquire whether the substance possessing 

 it has also the property of a dye. 



The investigations of Graebe, Liebermann, Kehrmann, 

 Kostanecki, Thiele and others have all tended to show that, 

 in general, the presence of double bonds in one form or 

 another is necessary for the production of colour. 



Of the simplest group of organic compounds, the hydro- 

 carbons, the greater number are colourless ; this is invariably 

 the case with the derivatives of methane, i.e. the saturated 

 hydrocarbons. The presence of one or two pairs of doubly- 

 linked carbon atoms is also insufficient to produce substances 

 absorbing light in the visible spectrum, but a certain class of 

 hydrocarbons containing three such ethylene linkages has been 

 discovered, all of which are highly coloured. 1 These hydro- 



1 Thiele, Ber. d. d. Chem. Ges. 33, 666 (1900). 



650 



