83 Dr. Schnnck 07i the Formation of Indigo-blue. 



more fugitive, and more liable to be acted on hj destructive 

 agencies, than the colouring principle of the Indiffoferts*." 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 am- 

 monia 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 muri- 

 atic 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. This change is readily eff'ected in the watery solution, 

 either by the addition of alkali, or by the temperature of the 

 solution rising to any great extent above that of the atmosphere. 

 When dissolved in alcohol or tether, on the other hand, the sub- 

 stance exhibits a much greater resistance to change than when 

 dissolved in water. I therefore soon abandoned all idea of ex- 

 tracting the plant with the latter menstruum, and had recourse 

 to alcohol and sether. I shall refrain from giving an account of 

 the numerous experiments I made, which led to no successful 

 issue, but shall briefly describe the three methods of preparation, 

 which were attended with more or less advantageous results. 



All three methods consist in extracting with alcohol or aether. 

 But, before commencing, it is necessary that the plant should 

 be thoroughly dried. The leaves alone being gathered, which 

 should only be done when not moistened with rain or dew, are 

 spread out in a thin layer in some warm, dry place, as, for in- 

 stance, near a stove or in a drying-room. All yellow or decayed 

 leaves should be rejected. When they are perfectly dry they are 

 carefully examined. All those leaves which during the drying 

 have become brown — a result which takes place when the heat 

 has been too great, — are cast aside, and only those are retained 

 which have a pale green colour. These must now be reduced to 

 a coarse powder, an operation easily eff'ected if the leaves be taken 

 while still warm, as they are then quite brittle; whereas if 

 * Perrottet, Art de VIndigotier, p. 110; 



