222 DR. EDWARD SCHUNCK ON INDIGO-BLUE. 



their lively greeu colour, except, occasionally, in certain 

 places where they have suffered injury from the bites of 

 insects or from other causes, and where blue spots make 

 their appearance — a phenomenon which I shall endeavour 

 to explain presently. It is nevertheless easy to show 

 that they contain a considerable quantity of what the 

 French call a " matiere colorable,^' i. e. a substance which, 

 though colourless in itself, yields colouring-matter by ap- 

 propriate treatment. A few leaves having been cut into 

 pieces and rubbed up with a little water in a mortar to a 

 thin paste, the mass is poured on a bit of calico, and yields, 

 by squeezing and kneading, a green muddy liquid which, 

 on the addition of a little sugar-of-lead solution, gives a 

 green flocculent precipitate containing the chlorophyll, al- 

 bumen, and other matters previously held in suspension. 

 The liquid filtered from this precipitate is clear and yellow, 

 and, on being mixed with sulphuric or hydrochloric acid 

 and left to stand for several hours, yields a deposit con- 

 sisting of tolerably pure indigo-blue. The amount of co- 

 louring-matter obtained in this way from Polygonum tinc- 

 torium is far greater than that which the same quantity of 

 woad-leaves grown in the same locality would produce, 

 proving that the yield is influenced not only by soil and 

 climate, but also by the peculiar nature of the plant. 



The isolation of the " matiere colorable " of Polygonum 

 tinctorium is still not a very easy task. The same pre- 

 cautions must be observed as in the case of Isatis tinctoria, 

 particularly as regards the evaporation of the solvents 

 employed, which must always be effected without applying 

 artificial heat. Unless some means are at disposal for 

 evaporating rapidly at the ordinary temperature, by means 

 of a current of air or otherwise, success is very uncertain. 

 The method formerly employed in preparing indican from 

 Isatis tinctoria was first tried. The leaves of the plant were 



