QUALITIES. _Form_, small round pieces, rather compact and heavy,
perforated in the middle; _Colour_, lively yellow with streaks of white;
it is easily pulverized, affording a powder of a bright buff-yellow
colour. CHEMICAL COMPOSITION. Gum, resin, extractive, tannin, gallic
acid, and a peculiar colouring matter, with traces of alumina and silex;
the white or flesh-coloured streaks pervading its substance, consist of
sulphate and oxalate of lime: according to the experiments of Mr. John
Henderson, there is besides a peculiar vegetable acid, to which he has
given the name of _Rheumic_ acid, but M. de Lassaignes has
satisfactorily proved that this is no other than the oxalic acid: the
purgative powers of the root appear to be intimately connected with its
extractive and resinous elements, but the subject is still involved in
considerable obscurity. SOLUBILITY. Water at 212° takes up 24 parts in
60, see _Infusum Rhei:_ by decoction, its purgative qualities are lost,
and it becomes more bitter and astringent; alcohol extracts 2·7 from 10
parts, (see _Tinct. Rhei_.) MED. USES. In this substance, Nature
presents us with a singular and most important combination of medicinal
powers, that of an astringent, with a cathartic property; the former of
which never opposes or interferes with the energy of the latter, since
it only takes effect when the substance is administered in small doses,
or if given in larger ones, not until it has ceased to operate as a
cathartic; this latter circumstance renders it particularly eligible in
cases of diarrhœa, as it evacuates the offending matter before it
operates as an astringent upon the bowels. It seems to act more
immediately upon the stomach and small intestines, and therefore in
relaxed and debilitated states of these organs, it will prove an easy
and valuable resource; it may, for such an object, be exhibited in
conjunction with alkalies, bitters, and other tonics. Its cathartic
property is most efficient when given in substance. It was formerly
supposed that by toasting rhubarb we increased its astringency, but this
process merely diminishes its purgative force, so that a larger dose may
be taken. The colouring matter of rhubarb may be detected in the urine
of persons to whom it has been exhibited; it does not however appear to
possess any specific powers as a diuretic. DOSE, grs. vj to x as a
tonic; ℈j to ʒss as a purgative; the operation of which is considerably
quickened by the addition of neutral salts; the super-sulphate of potass
forms also a very useful adjunct, and its acidulous taste completely
covers that of the rhubarb. _Form. 83, 85._ Its powder, when sprinkled
upon ulcers, is found to promote their healthy granulation. OFF. PREP.
_Infus: Rhei_. L.E. _Vinum Rhei Palmati_. E. _Tinct. Rhei_. L.E.D.
_Tinct. Rhei. comp_. L. _Tinct. Rhei cum Alöe_. E. _Tinct. Rhei cum
Gentian_. E. _Pil. Rhei comp_. E.