88 Mr. Crum on Indigo. [Feb, 



what is still more wonderful, by placing this amalgam in a solu- 

 tion of nitrate of silver, he obtained crystals in the form of an 

 artichoke, which were an alloy of the metal of indigo with silver. 

 In consequence of these statements, I made several attempts to 

 amalgamate sublimed indigo, as well by the process of Doberei- 

 ner, as by others which I thought more likely to succeed, but in 

 vain. I could in no case perceive the least alteration in the 

 fluidity of the quicksilver. 



Action of Sulphuric Acid upon Indigo. 



When indigo is digested in concentrated sulphuric acid, it is 

 well known to suffer a remarkable change, being converted into 

 a peculiar blue substance, entirely different from indigo, with 

 which the Saxon blue is dyed. 



This substance has been so little attended to by chemists that 

 no one has yet thought of giving it a separate name. I shall 

 venture to propose for it that of cerulin, from the colour of its 

 solution. 



The mixture of the blue substance with sulphuric acid is a 

 semifluid, which requires a considerable quantity of water to 

 dissolve it. When potash is added to this solution, previously 

 filtered, a deep-blue precipitate is formed. I was surprised, 

 however, when making the experiment, to find the precipitate 

 as plentiful before one-fourth of the acid was saturated, as when 

 the solution was made altogether neutral. To another portion, 1 

 added potash previously saturated with sulphuric acid, and found 

 the same precipitate formed as with potash alone.* I threw the 

 precipitate upon a filter, and washed it with water, in order to 

 examine whether it was the pure colouring matter that had 

 separated. The first washing did not take away much of the 

 precipitate; the next, however, sensibly diminished it; but with 

 the third portion of water, it almost wholly disappeared. 



In order to discover the cause of this increased solubility, I 

 made a saturated solution of sulphate of potash in water, and, 

 putting into it a little of the blue pulpy substance that remained 

 upon the filter, I agitated it thoroughly. The solution remained 

 altogether colourless. I found the same to be the case when 

 the muriate, the acetate, or any other salt of potash, was 

 employed. Alcohol also, and ether refused to dissolve this 

 substance. Put into pure water, however, it immediately dis- 

 solved, forming the same deep blue solution that had passed 

 through the filter. 



• That the solution of indigo in sulphuric acid is precipitated by neutral salts is not 

 a new fact. Berthollet (Art of Dyeing, ii. 50) says, that the " fixed alkalies saturated 

 with carbonic acid," as well as " alcohol, saturated solutions of alum, sulphate of soda, 

 or other salts containing sulphuric acid," form precipitates in this solution. My expe- 

 riments show, I think, that the effect does not depend upon the presence of sulphuric 

 acid in the precipitant ; that some sulphates have no such effect ; and that alcohol does 

 not precipitate the original solution at all. 



