1. _Rheo-tannic Acid_, C₂₆H₂₆O₁₄, a yellowish powder abundantly present
in rhubarb, soluble in water or alcohol, not in ether. Its solutions
produce blackish-green precipitates with persalts of iron, and greyish
ones slowly turning blue, with protosalts of the same.
2. _Rheumic Acid_ (_Rheumsäure_), C₂₀H₁₆O₉, obtained as a reddish-brown
powder, by boiling rheo-tannic acid with a dilute mineral acid, a
fermentable sugar being developed at the same time. Rheumic acid
exhibits nearly the same reactions as rheo-tannic acid, but is very
sparingly soluble in cold water. It partly pre-exists in rhubarb.
3. Neutral _colourless_ substance, sparingly soluble in hot water, and
separating from the latter in prismatic _crystals_ of the formula
C₁₀H₁₂O₄; no name has yet been given to it. A “white crystalline resin”
(and a dark brown crystalline resin) has been isolated in 1878 by
Dragendorff.
4. _Phæoretin_, C₁₆H₁₆O₇, agreeing with the substance thus named by
Schlossberger and Döpping. It is a brown powder, soluble in alcohol or
in acetic acid, but not in ether, chloroform or water.