152 IlESIXS, BITTE B BBINCIPLES, ETC. 



Usnic acid, which occurs in yellow crystals, yields a colourless 

 ammonium salt. 



§ 158. Orcin and Betaorcin. Estimation of Orcin. — Orcin and beta- 

 orcin, which have already been mentioned as products of the 

 decomposition of certain constituents of lichens, and which some- 

 times occur ready formed in plants, can be obtained in colourless 

 acicular crystals soluble in water, alcohol, and ether. Exposure 

 to light colours them reddish; alkalies, chlorinated lime and ferric 

 chloride, violet. By the action of ammonia and air orcin yields a 

 blue colouring matter, whilst, under the same conditions, beta- 

 orcin gradually turns red. Orcin melts at 58°, betaorcin at a 

 temperature above 109°. 



Eeymann estimates orcin in lichens by titrating with bromine- 

 water,^ by which monobromorcin is first produced and subse- 

 quently converted into tribromorcin. To the solution of orcin 

 in a stoppered bottle titrated bromine-water is added till the 

 precipitate has assumed a yellowish colour and excess of bromine 

 is present, which is then estimated by iodide of jDotassium and 

 hyposulphite of soda. The amount of orcin j)resent is calculated 

 from the equations : 



C^HsO^ -f Br. = HBr + C^H^BrO^ 



and 



C^H-BrO^ -h 2Br, = 2HBr -F C^H^Br^O 



TANNIC ACIDS. 



§ 159. Constitution. — In estimating tannic acids an error has 

 generally been committed in overlooking too completely the 

 chemical differences existing between the various substances that 

 have received this name. It has usually been considered sufficient 

 to determine quantitatively the value of a reagent in terms of 

 gallotannic acid, the tannin most easily procurable, and to apply 

 the results thus obtained to the estimation of other tannins. 

 This would be admissible under the assumption that all tannins 

 possessed approximately identical equivalent weights and pro- 

 duced nearly identical chemical eff'ects. But it has ali'eady been 

 shown in § 52 that such is not the case. It will be sufficient 

 here to repeat that tannins exist which do not allow of com- 

 parisons with one another, even with regard to their constitution. 

 At present many tannic acids may he assumed to be glucosides, 



1 Ber. d. d. chem. Ges. viii. 790, 1875 (Journ. Chem. Soc. xxvii. 1293). 



