378 Prof. H. E. Armstrong on Loiv-Temperature Research 

 OH 



Quinone 



form an unsymmetrical light absorbing system. Most coloured 

 substances appear to contain such a " quinonoid " system ; even a 

 simple substance such as iodoform, CHI3, which has a yellow colour, 

 may be regarded as a compound of quinonoid type, since iodine has 

 a tendency to function as a tervalent element, so that each iodine 

 atom in iodoform has two affinities potentially free. Normally, the 

 single quinonoid system has a pale colour — a more or less pronounced 

 yellow — but if additional " absorbing centres " are developed in the 

 molecule, the colour g];ows in intensity and is especially intense in 

 cases in which two or more such systems are associated. 



Mercuric iodide is another case in point : in the solid state this 

 compound is either yellow or scarlet ; in the gaseous state, however, 

 it is said to be colourless and when dissolved in certain solvents it 

 forms colourless solutions. According to the quinonoid hypothesiSj 

 a molecule of the formula HgL should be colourless, like that of 

 methylene iodide, CH^Io ; but if several such molecules were con- 

 joined, thus 



Hg 



^ 



Hgl2 



l2Hg 



a system corresponding to that of quinone would be formed. 



It is not improbable that the yellow colour of solid iodoform is 

 that of a complex such as 



I = I 

 HCI=nCH 



I - I 



Moreover, the blue colour of water, of oxygen and even that of 

 ozone, in which Faraday displayed such special interest, is not 

 improbably conditioned by the presence of molecules of a more 



