916 NEUTRAL COLOURING MATTERS. 



sulphuric acid or oil of vitriol, when slightly heated with 

 that acid in the proportion of 1 to 8, without any evolution of 

 sulphurous acid, and forms a semi-fluid mass, popularly known 

 as sulphate of indigo, which dissolves in a considerable quantity 

 of water, of an intense blue colour. The products of this action 

 were first examined by Mr. Crum, whose results form the basis 

 of M. Dumas' later investigations. To convert blue indigo 

 into sulphindylic acid, the latter chemist recommends the 

 digestion of the indigo in a large quantity, not less than 15 

 parts, of concentrated oil of vitriol, for three days in a bath 

 maintained at a temperature of 1 20 or 1 40. The solution is 

 afterwards diluted with water and filtered, although when these 

 precautions are attended to, little or nothing insoluble remains 

 on the filter. To the limpid liquid a strong solution of pure 

 acetate of potash is added, and the precipitate which falls is 

 washed with acetate of potash, in which salt the sulphindylate 

 of potash is insoluble although soluble in water ; the acetate of 

 potash remaining in the precipitate is got rid of by diffusing 

 the latter through a quantity of ordinary alcohol and filtering 

 again. The blue matter remaining when well dried at 212, 

 gave by analysis, KO-f-C 16 H 4 NO,S 2 O 6 ; conducting to the 

 following formula for hydrated sulphindylic acid, HO-f 

 C 16 H 4 NO,S 2 O 6 . Blue indigo thus appears to lose HO in the 

 formation of sulphindylic acid. Sulphopurpuric acid, remains 

 upon the filter in the preparation of sulphate of indigo, when 

 8 or 10 parts only of sulphuric acid have been employed to 1 of 

 indigo. It is drained, and washed with diluted hydrochloric 

 acid, till the washings are colourless and free from sulphuric 

 acid. It is then carefully dried about 392 (200 centig.). 

 This purple acid gave by analysis, C 32 H 10 N 2 O 4 -f 2SO 3 . The 

 sulphopurpurate of potash is obtained by dissolving the acid in 

 water, and adding acetate of potash to the liquid ; it precipitates 

 in purple flocks, which should be washed first with a solution 

 of acetate of potash, and then with alcohol. Its composition 

 indicates that the atom of indigo C 16 H 5 NO 2 takes the isomeric 

 state C 32 H 10 NO 4 , to constitute sulphopurpuric acid. M. Dumas 

 institutes the following comparison between the indigo and 

 benzoyl compounds : 



