664 SCIENCE PROGRESS 



that to produce a colour-change a rearrangement of the 

 molecule is necessary. This explains the fact that the alkyl and 

 acyl derivatives of the nitrophenols, X ■ OCH 3 and X ■ O ■ COCH 3 

 are colourless, on the ground that they have an equal action in 

 rendering the hydrogen atom immovable — i.e. preventing an 



isomerisation from X ■ OH to X<^ . Similar explanations are 



given to the colour changes of the nitranilines, violuric acid and 

 similar oximido ketones, the dinitro and trinitro paraffins, etc. 



As required by the dynamic theory, Hantzsch contends that 

 the " auxochromic " value of the —OH group is exactly equal to 

 that of the methoxyl, and he points out that Baly's 1 work on 

 absorption-spectra confirms this view. The absorption-spectra 

 of /flra-nitrophenol and /wra-nitroanisol fall together in the 

 remote ultra-violet. 



An indirect proof of the quinonoid nature of the nitrophenols 

 has been given by the discovery of the coloured mercury-««- 

 nitrophenols 2 which contain mercury in a closed chain. From 

 their behaviour they must have the formula — 



O 



In a paper entitled " Colour and Constitution of Acids, Salts 

 and Esters," 3 Hantzsch extends his views on the dynamic theory 

 of colour to include the classes of compounds named. For most 

 of these substances the second tautomeric forms have not yet 

 been isolated, but great differences have been observed in the 

 colour and behaviour of closely related derivatives, chiefly salts. 

 In the case of salicyl aldehyde, the true phenol is colourless 



r tt /OH(R') _ . 



\HO ' 10ns anc * t ^ ie a ^ an salts are, however, 



coloured, showing a transformation into C C H 4 / Z. , whilst 



xCH ' OM 



the ammonium salt again is colourless, i.e. C 6 H 4 <^^ 4 . 



N CHO 



In the case of substances of acidic nature, Hantzsch 



1 Journ. Chem. Soc. 89, 518 (1906). 



2 Ber. d. d. Chem. Ges. 39, 1105 (1906). 



3 Ibid. 39, 3080 (1906). 



