CHRYSOPHANIC ACID. 409 



When heated with zinc-dust, alizarin is converted 

 into anthracene ; when oxidized with nitric acid, phta- 

 lic acid is the product. Excellent dye. 



Chrysophanic acid (Parietic acid, Rheic acid), 

 C^H^OH^O 2 or C 14 H 8 (OH) 2 2 (isomeric with alizarin or 

 an analogous derivative of anthracene dihydride). Is 

 contained in the lichen Parmelia parietina ; in rhubarb 

 (the root of various species of Rheum) ; and in senna 

 leaves (from Cassia lanceolate and Cassia obovata). Can be 

 readily obtained from these plants by extracting with 

 caustic potassa, precipitating with hydrochloric acid 

 and recrystallizing from chloroform. Yellow, lustrous 

 prisms ; fusing point, 162 ; partially sublimable ; al- 

 most insoluble in water, but slightly in alcohol, easily 

 soluble in ether. Soluble in alkalies, the solutions 

 being red. Heated with zinc-dust it is converted into 

 anthracene. 



Chrysammic acid (Tetranitro- Dioxyanthraqui- 

 none), C U H 2 (N0 2 ) 4 (OH) 2 2 . Is formed by warming 

 crysophanic acid and aloes (see Aloin) with concentrat- 

 ed nitric acid. Golden yellow, lustrous laminae, very 

 similar to lead iodide. But slightly soluble in water. 

 Strong bibasic acid. Reducing substances (hydriodic 

 acid, zinc and dilute sulphuric acid, potassium sul- 

 phide) convert it into hydrochrysamide, C 14 H 2 (]N"H 2 ) 3 

 (1TO 2 XOH) 2 2 , a body that forms indigo-colored nee- 

 dles; sublimable, when carefully heated ; insoluble in 

 water. 



Purpurin (Trioxyanthraquinone), C 14 H 8 5 = 

 C 14 H 5 (OH) 3 2 . Is contained in old madder, and is also 

 sometimes formed as a by-product in the artificial pre- 

 paration of alizarin. Reddish-yellow prisms. Easily 

 fusible and sublimable. Somewhat more easily solu- 

 ble in water than alizarin, easily soluble in alcohol, 

 ether, and alkalies, the solutions having a red color. 

 It gives purplish-red precipitates with lime- and 

 baryta-water. When heated with zinc-dust it is con- 

 verted into anthracene. 

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