RADIX RHEI. 41)9 



As to the contents of the white cells, they are loaded either with 

 starch or tufted crystals of oxalate of calcium, the amount of the latter 

 being especially liable to variation. Scheele, after having discovered the 

 oxalic acid, pointed out in 1784 that the crystals under notice consist 

 of that acid in combination with lime ; he was the first to point out the 

 true composition of those crystals which are of so wide a distribution 

 throughout the vegetable kingdom. The medullary rays contain the 

 substances peculiar to rhubarb, but none of them occur in a crystalline 

 state. 



Chemical Composition. — The active constituent of the root has 



long been supposed to reside in the yellowish red contents of the medul- 

 lary rays. Schrader as early as 1 807 prepared a Rhuharb-Bitter, to which 

 he attributed the medicinal powers of the drug. Since then several sub- 

 stances of the same kind have been separated by various methods, and 

 described under difterent names: such are the Rhabarberstqff of Tromxns- 

 dorff, the Eheumin of Hornemann, the Bhabarberin of Buchner and 

 Herberger, the Bhubarb-Yelloiv or Bhein, and the BkabarbaHc Acid of 

 Brandes. 



Schlossberger and Dopping in 1844 first recognized among the above- 

 named substances a definite chemical body named Ghrysophan or Chryso- 



phanic Acid, C"H' -J /r)fT\2^\ which had been found in 1843 by Roch- 



leder and Heldt in the yellow lichen, Pai^melia parietina. It partly 

 forms the 3'ellow contents of the medullary rays of rhubarb, and when 

 isolated crystallizes in golden yellow needles or in plates. It dissolves 

 in ether, alcohol, or benzol ; though scarcely soluble in water, it is 

 nevertheless extracted from the root to some extent by that solvent, 

 probably by reason of some accompanying substance. Alkalis dissolve 

 it, forming fine dark red solutions. Ghrysophan, C^'ff"0^ is a deriva- 

 tive of anthracene, C"!!"', and closely allied to alizarin, C"H®0\ 



By precipitating alcoholic solutions of extract of rhubarb with ether, 

 Schlossberger and Dopping obtained, together with chrysophan, resinous 

 bodies which they named Aporetin, Phceoretin and Erythroretin. 



De la Rue and Miiller (1857) extracted from rhubarb, in addition to 

 cTirysophan, an allied substance, Eniodin, which crystallizes in orange- 

 coloured prisms, sometimes as much as two inches long. Its constitu- 



tion was subsequently found to agree with the formula C"H^ \ ((\tj\^^'- 



Kubly (1867) has obtained from rhubarb the following con- 

 stituents : — 



1. Bheo-tannic Add, C^^H^O^^, a yellowish powder abundantly pre- 

 sent 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. Bheumic Acid (Bheumsdiire), C^^'H^^O^, obtained as a reddish- 

 brown powder, by boiling rheo-tannic acid with a dilute mineral acid, 

 a fei-mentable 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 C'^H^^qj. 



