134 HESIXS, BITTEE PRINCIPLES, ETC. 



Schunk and Eömer,i indirect j)roof may be obtained by taking 

 advantage of the unequal affinity of the two substances in alkaline 

 solution for atmospheric oxygen. A solution in caustic soda is 

 exposed to the air until it has become almost colourless and ceases 

 to show the spectrum of purpurin after the addition of more 

 alkali. By acidifying with hydrochloric acid and agitating with 

 ether, the alizarin can be extracted, redissolved in alcoholic 

 potash, and tested spectroscopically. 



The scale on Plate I. corresponds to that described in § 20. I 

 shall subsequently come to speak of the spectra of chloi'ophyll, 

 haämatoxylin, and some other colouring matters (partly taken 

 from Vogel) also figured on the same plate. 



Purjmrin forms orange-red needles, melting at 253°, and soluble 

 in boiling water and alcohol, but more freely so in ether, bisul- 

 phide of carbon, and boiling benzene. Aqueous solutions of alum 

 dissolve it, forming yellow liquids with green fluorescence ; with 

 dilute aqueous alkalies purple solutions are obtained ; it dissolves 

 Avith difficulty in alcoholic soda, and is precipitated by lime- and 

 baryta-water. 



The erijtlirosderotin, or sclererythrin, isolated by Podwissotzky 

 and myself" from ergot is, I think, possibly identical with, or 

 closely allied to, jjurpurin. 



Alizarin is generally considered to be produced from a glucoside, 

 ruberythric acid, and not to occur ready-formed in the madder 

 l^lant ; ruberythric acid is possibly itself a prod^^ct of the decom- 

 position of nihian. The latter is said to be soluble in hot water 

 and in alcohol ; from aqueous solution it is not precipitated by 

 solution of alum or lead salts, but probably it has not yet been 

 obtained in a state of purity. Boiling solutions of alkalies dis- 

 solve rubian Avith jDroduction of a red colouration and formation 

 of alizarin, rubiretin, verantin, rubiadin and sugar. Boiling 

 dilute acids induce a similar decomposition, whilst Avith cold 

 dilute alkali it yields rul)ianic acid. 



liuberijthric acid is freely soluble in hot Avatcr, in alcohol, and 

 in ether. It crystallizes in yellow silky prisms, and forms blood- 

 red solutions Avith alkalies. Basic acetate of lead precipitates it 

 as a vermilionrcd powder. Boiling Avitli dilute acid resolves it 



^ Ber. d. d. chem. Ges. x. 175, 1877 (.Jouin. Chem. Soc. xxxi. 664). 



2 Archiv f. exper. Patholog. und Pharmakologie, vi. 154, 1876 (Pharm. 

 Joum. and Trans. [3], vi. 1001, viii. 106). Sitz-13cr. d. Dorpater Naturf. Ges., 

 392, 1877. 



