FORMATION OF INDIGO-BLUE. 189 



now only remained to separate it from the other constituents 

 of the plant and ascertain its properties and composition. 

 But, though I arrived at the conclusion just stated without 

 any great difficulty, I found that the isolation and prepara- 

 tion in a state of purity of the substance whose existence 

 had been indicated by these experiments, constituted a pro- 

 blem of no easy solution. I soon discovered that this body 

 is extremely liable to decompositon, so much so, as com- 

 pletely to justify the assertion of an author, who in speaking 

 of the difficulties of the manufacture of indigo, says that 

 " nothing is more fugitive and more liable to be acted on by 

 destructive agencies, than the colouring principle of the 

 indigoferse." * The continued action of water, even at a 

 moderate, but especially at a high temperature, as well as 

 that of alkalies, I found to induce a complete change in the 

 body which I was endeavouring to isolate. The fact of its 

 being completely precipitated from the watery extract of 

 "woad by means of acetate of lead and ammonia, and of the 

 lead compound being readily decomposed by means of a 

 current of carbonic acid gas, seemed at first to lead to an 

 easy method of preparation. But on extracting a large 

 quantity of the plant with cold water, adding sugar of lead 

 to precipitate the albumen and green colouring matter, and 

 then adding ammonia to the filtered liquid, I found that the 

 precipitate produced by ammonia, though the alkali was not 

 used in excess, contained the substance in an altered state. 

 This change, the nature of which I shall treat of presently, 

 is readily indicated by boiling some solution of the substance 

 in water with sulphuric or muriatic acid, when it will be 

 found that black flocks are deposited, which when treated 

 with boiling alcohol impart to the latter no trace of blue, 

 but a pure brown colour, a great part of the flocks remaining 

 undissolved by the alcohol in the shape of a black powder. 

 <rhis change is readily effiected in the watery solution, either 

 • Perrottet, Art de riodigotier, p. 110. 



