192 MR. E. SCHUNCK ON THE 



evaporating the yellow liquid as before, either spontaneously 

 in the air or in vacuo. If purification should be necessary, 

 it is effected, as in the preceding case, by means of oxide 

 of copper. I endeavoured to modify this method by agi- 

 tating the etherial extract with cold water, but without any 

 advantage resulting, as the substance was not entirely re- 

 moved from the ether by means of water, a considerable 

 quantity still remaining dissolved in the ether. 



III. The pounded woad leaves are extracted in a dis- 

 placement apparatus as before with cold alcohol. To the 

 green alcoholic extract there is added an alcoholic solution of 

 acetate of lead, which produces a pale green precipitate, the 

 precipitation being then completed by the addition of a little 

 ammonia. The precipitate which is bulky is placed on a 

 filter and washed with cold alcohol, until the percolating 

 liquid instead of being dark green is only light green, and 

 the excess of acetate of lead and ammonia have been re- 

 moved. It is then suspended in water and a current of 

 carbonic acid gas is passed through the liquid. The pre- 

 cipitate gradually becomes paler in colour and at last almost 

 white, and loses considerably in bulk, while the liquid ac- 

 quires a yellow colour. The latter being filtered, sulphu- 

 retted hydrogen is passed through it to precipitate a little 

 oxide of lead contained in it, and being again filtered is 

 evaporated as before either in the air or in vacuo over sul- 

 phuric acid. 



Of these three methods I prefer the last, as being more 

 expeditious than the first and less costly than the second. 

 One precaution must not be forgotten, that of never in the 

 last instance attempting to evaporate the solution at a higher 

 temperature than the ordinary one. If the attempt be made, 

 the substance will undergo an entire change, as I have re- 

 peatedly discovered to my cost. This change consists in 

 the substance taking up the elements of water. It takes 

 place as well in vacuo as in the air. I have no reason to 



