92 Mr. Crum on Indigo. [Feb. 



galls. It appears that cerulin acts a similar part, at least with 

 the sulphuric salts. I am not aware, however, that any sub- 

 stance has been hitherto described by chemists which possesses 

 the property I have found in the one here treated of, that of 

 dissolving in pure water, and refusing to do so in neutral saline 

 solutions which produce no change upon it. But in common 

 life, some idea of this singular fact seems to have been long 

 acted upon. Those who are in the habit of washing printed 

 gowns, particularly dark ones, of colours not very permanent, 

 always rince them in a solution of common salt, or at least in 

 very hard water, before they hang them up to dry. The salt, 

 they say, fixes the colour, and prevents it from spreading out 

 into the white, which it always does when they are suffered to 

 dry in a cool place, without such immersion. 



On the Phenomena which are exhibited during the Formation of 



Cerulin. 



Some of the phenomena which attend the action of sulphuric 

 acid upon indigo, have been noticed by different chemists. 

 Bergman, in 177b', observed, that when indigo in powder was 

 sprinkled upon sulphuric acid, greenish clouds were produced, 

 which became blue by the addition of a drop of water.* He 

 added that the same effect was produced, but more slowly, with- 

 out water. Haussman, of Colmar.f observed, that the acid in 

 contact with indigo, became at first greenish-yellow, then deep- 

 green, and at last blue. That gentleman remarked also, that 

 the " effervescence and disengagement of sulphurous vapours, 

 always observed in making the blue compound, leave no room 

 to doubt that the acid exerts an action upon the particles of 

 indigo, and that we should be wrong in considering this as a 

 mere solution of indigo unaltered." Berthollet, in his excellent 

 work on Dyeing,;]; considers the change that takes place a spe- 

 cies of combustion ; the sulphuric acid furnishing the indigo 

 with oxygen, and thereby being converted into sulphurous acid. 

 Dr. Bancroft, whose work appeared soon after that of Berthol- 

 let, conceived the solution to be oxygenated indigo combined 

 with sulphuric acid. Hence he gave it the name of sulphate of 

 indigo.§ 



Such, as far as my information goes, is the extent of our know- 

 ledge, or rather our opinions, upon this subject. I shall state 

 what appearances I have myself observed during this process. 



When indigo is put into sulphuric acid, it is dissolved, and the 

 acid assumes a yellow colour. When this solution is dropped 

 into water, it becomes instantly blue ; but the substance so pro- 

 duced is by no means the same as that which is formed after 



* Opusc. torn. v. p. 7. Edit. 1788. 

 •f- Journal de Physique for March, 1788. 

 j Vol. ii. pp. 50, 66, English translation. 

 \ On Permanent Colours, pp. 104, 133. 



