DIHYDRIC PHENOLS 197 



red coloration ; tannic and gallic acids yield a yellow tint, 

 phloroglucin red, and so on. 



The material to be examined is merely exposed to the 

 vapour of ordinary sweet spirits of nitre which contain 4 per 

 cent of the ethyl nitrate, or a 20 per cent alcoholic solution 

 of the commercial nitrous ether may be employed. If the 

 latter method be used the time required for full precipitation 

 is considerably less, 



CHEMISTRY. 



We have as yet comparatively little certain knowledge 

 concerning the chemical constitution of even the simplest 

 tannins. Thus, for example, although ordinary tannic or 

 gallotannic acid is generally regarded as an anhydride formed 

 by the removal of one molecule of water between two mole- 

 cules of gallic acid, according to the equation 



2C,H,0g - H^O = Ci.HioOy 

 Gallic acid Gallotannic acid 



there are even now, as will be seen below (p. 212), differences 

 of opinion with regard to the exact position from which the 

 two atoms of hydrogen and one of oxygen have been removed. 



While the composition of the various classes of tannins 

 of course varies considerably, they are probably all more or less 

 complex derivatives of gallic or ellagic acids, or their methy- 

 lated derivatives, or are condensation products of these or simi- 

 lar acids, with various phenolic substances. 



In view of these facts the classification and properties of 

 the tannins will be more easily understood if preceded by a 

 brief description of certain relatively simple phenolic sub- 

 stances from which the complex tannins are built up (p. 217). 

 The substances include the following : — 



I. The Dihydric phenols — pyrocatechol, resorcinol and 

 hydroquinone. 



OH OH OH 



°" a . 



OH 



Pyrocatechol Resorcinol Hydroquinone 



