MR. SCHUNCK ON RUBIAN AND ITS PRODUCTS OF DECOMPOSITION. 437 
taining rubian ; but the presence of chlorogenine is also indicated by its turning dark 
green when boiled with the addition of sulphuric or muriatic acid. Hence it follows 
that chlorogenine, though it is not thrown down by basic acetate of lead when pre- 
sent alone in a solution, is still in part precipitated thereby when rubian is present at 
the same time. The same circumstance takes place with other precipitants. \ 
After numerous experiments I discovered a property of rubian, which is perhaps 
more characteristic of it than any other, and that is the remarkable attraction which 
is manifested by it towards all substances of a porous or finely-divided nature, and it 
was this property by means of which I was at length enabled to obtain it in a state 
of purity. If to a watery extract of madder a quantity of protochloride of tin be 
added, a light purple lake is precipitated. Most of the rubian remains in the solu- 
tion, which still retains its yellow colour and bitter taste. If, however, after filtering, 
sulphuretted hydrogen be passed through it, then, provided the quantity of tin still 
in solution be sufficiently large, the sulphuret of tin, at the moment of precipitation, 
carries down the whole of the rubian, and the solution loses its bitter taste and the 
greater part of its yellow colour. The whole of the chlorogenine remains in solution, 
and may easily be detected in the filtered liquid by means of acids. If the sulphuret 
of tin, after being collected on a filter and well washed with cold water until the per- 
colating liquid no longer gives a green colour on being mixed with acid and boiled, 
be treated with boiling alcohol, a yellow solution is obtained, which on evaporation 
gives pure rubian, without any admixture of chlorogenine, in the shape of a dark 
yellow, brittle substance. The same effect is produced by sulphuret of lead. If 
sugar of lead be added to an extract of madder, a dark reddish-brown precipitate 
falls, the liquid still containing the rubian of the extract, as seen by its deep yellow 
colour and bitter taste. If sulphuretted hydrogen be now passed through the filtered 
liquid, a great part of the rubian goes down with the sulphuret of lead, and may 
again be separated from it by means of boiling alcohol. That this action of the sul- 
phurets on rubian depends very much on their state of division, and is therefore 
mainly of a mechanical, and not chemical nature, is proved by the fact, that the sul- 
phurets of tin and lead, if prepared by precipitation from solutions of salts in water, 
and then allowed to settle and repose for some time before being added to a watery 
extract of madder, remove far less rubian from it than they do, if they are formed in 
the extract itself, whence it follows that it is only in the minute state of division, in 
which they exist at the moment of precipitation, before the particles have time to 
cohere, that these sulphurets exert any great attraction for rubian. That they do 
however combine with some portion of the rubian is proved by the fact, that the 
power of dyeing in an extract of madder is very much diminished by adding to it 
sulphuret of tin or lead, previously precipitated. Of the two sulphurets, the sul- 
phuret of tin, which is always precipitated in much finer particles than the other, is 
by far the most powerful absorbent of rubian. If equivalent quantities of proto- 
chloride of tin and acetate of lead be added to equal measures of watery extract of 
MDCCCLI. 3 L, 
