| C₁₄H₄ |  | CH₃ | O₂. |
| (OH)₃ |
Kubly (1867) has obtained from rhubarb the following constituents:—
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.
5. Chrysophan, described above.
According to Dragendorff (1878) mucilaginous matters occur in the different varieties of rhubarb to the amount of from 11 to 17 per cent. He states them to consist of mucilage (properly so called), arabic acid, metarabic acid and pararabin, and moreover enumerates also pectose among the constituents of the drug.
Small quantities of albuminoid substances, malic acid, fat and sugar have also been met with in rhubarb. As to its mineral constituents, their amount is exceedingly variable. Two samples of good China Rhubarb dried at 100° C. and incinerated, yielded us respectively 12·9 and 13·87 per cent. of ash. Another sample, which we had particularly selected on account of its pale tint, afforded no less than 43·27 per cent. of ash. The ash consists of carbonates of calcium and potassium. English rhubarb from Banbury (portions of a large specimen) left after incineration 10·90 per cent. of ash.
From a practical point of view the chemical history of rhubarb is far from satisfactory, for we are still ignorant to what principle the drug owes its therapeutic value, or what are the pharmaceutical preparations in which the active matter may be most appropriately exhibited. Chrysophan is said to act as a purgative, but less powerfully than rhubarb itself.