157 
“ On Indigo-blue from Polygonum tinctorium and other 
plants,” by Edward Schunck, Ph.D., F.R.S. 
The author, after referring to his investigation of Isatis 
tinctoria, the common woad plant, the results of which 
were communicated to the Society many years ago,* pro- 
ceeded to give an account of some experiments he had 
recently made with Polygonum tinctorium, a plant em- 
ployed by the Chinese for the manufacture of indigo, his 
object being to ascertain whether the colouring matter is 
contained in this plant in the same form as in the Isatis, 
viz. as a glucoside. His experiments led to the conclusion 
that the leaves of P. tinctorium contain a substance which 
cannot be distinguished from the indican of the woad plant. 
It is amorphous, soluble in water, alcohol, and ether, and 
by the action of acids is decomposed into indigo-blue and a 
substance giving the reaction of glucose, probably indig- 
ucine. When its watery solution is boiled or left to stand 
for some time, it undergoes a complete change, and then no 
longer yields indigo-blue by decomposition with acids, but 
indigo-red and other products, indican, as formerly shown, 
undergoing a similar metamorphosis under the same 
circumstances. 
The author recommends for the preparation of this sub- 
stance the following process : — The leaves of the plant having 
been carefully dried, are ground to powder and extracted 
with spirits of wine. The green alcoholic extract is evapo- 
rated at the ordinary temperature, a current of air being 
employed to assist evaporation. After evaporation of the 
alcohol there is left a brown watery liquid, which is filtered 
from the deposited chlorophyll and fatty matters, and mixed 
with acetate of lead solution. This gives a copious dirty 
yellow precipitate, which is filtered off. Basic lead acetate 
added to the filtrate produces a primrose-yellow precipitate, 
which is filtered off, washed with water, then with alcohol, 
* Memoirs, 2nd ser., XII. p. 177, and XIV. p. 181, 
