1310 INDOXYL-PIGMENTS. 



the colouring substance being known as melanin. 1 The urine 

 of patients suffering from these tumours is either dark brown 

 or black when voided or speedily assumes this colour after brief 

 exposure to the air or by the action of nitric acid or other oxi- 

 dizing agents, the pigment to which the colour is due being 

 apparently identical with that present in the tumour. This 

 action of oxidizing agents indicates that here also, as in the 

 case of other urinary pigments, there is primarily some chrorao- 

 genic forerunner (melanogen) of the actual pigment. This 

 chromogen as also the fully formed pigment may be partially 

 precipitated by baryta-water, the remainder being precipitable 

 by the addition of normal lead acetate. The purified pigment 

 is partly insoluble, partly soluble in acetic acid of 50 75 p.c. 

 Of these portions the former when dried is a brownish-black 

 amorphous powder, insoluble in either water, alcohol, ether, 

 chloroform or dilute (mineral) acids, but readily soluble in 

 alkalis. The latter has been obtained in too small amounts to 

 admit of complete investigation. Some melanins may con- 

 tain iron (-2 p.c.) and some a considerable amount of sulphur 

 (9 p.c.) and none shew &ny absorption bands when their solu- 

 tions are examined spectroscopically. 



When melanotic urines are treated with oxidizing agents 

 or solutions of ferric chloride, they yield, according to the con- 

 centration of the reagent, either a dark brown cloudiness or 

 else a black precipitate soluble in excess of the precipitant: 

 this test is both delicate and characteristic. 



6. Indoxyl-pigments. 



Of the total indole formed in the alimentary canal, a portion 

 is excreted with the faeces, while the remainder is absorbed and 

 reappears in the urine united with potassium as ethereal com- 

 pounds of indoxyl with either glycuronic acid (p. 1220) or 

 sulphuric acid (p. 1281), the latter being known as urinary 

 indican. When warmed with hydrochloric acid these com- 

 pounds are decomposed, yielding indoxyl and the potassium 

 salt of the corresponding acid. If the decomposition is effected 

 in the absence of oxygen, the indoxyl may be in part gradually 

 changed into an amorphous reddish substance, indigo-red, which 

 is insoluble in water, but yields a red solution when dissolved 

 in alcohol, ether or chloroform. These solutions shew no cer- 

 tainly characteristic absorption bands. In presence of oxygen 

 and with most certainty by the action of an oxidizing agent, the 

 indoxyl is readily converted into indigo-blue, whose properties 

 and solubilities have been already sufficiently described(p. 1282). 



1 The name melanin is more usually applied as a generic title for the dark 

 brown or black pigments such as occur in the hair, epidermis, retinal epithelium, 

 choroid, etc. 



