of

111

818 ELEMENTS OF MATERIA MEDICA.

tion of carbonate of soda, forms, with nitrate of silver, a white precipitate (oxalosilver), which explodes when heated.

Chemical Characteristics. —If the powder of rhubarb be heate^ ^a glass capsule over a lamp, an odorous yellow vapour ( rhabaroei ^pyro-rhabarberic acid) is obtained, which communicates a red colota solution of caustic potash. The aqueous infusion of rhubarb * 0 ^.with the sesquicliloride of iron, a green compound ( tannate oj *with a solution of gelatin, a copious yellow precipitate ^

gelatin ), which is dissolved on the application of heat, or by the adof an excess of gelatin ; with a solution of sulphate of quina, a yed jprecipitate ( tannate of quina) ; with the alkalies (potash, soda,ammonia) a red-coloured solution ( soluble alkaline rhabarberates )> ^

lime-water, a reddish precipitate ( rhabarberate of lime) ; with the a ^(the acetic excepted), precipitates (composed of rhabarberic acidprecipitant) ; and with various metallic solutions (as of acetate ot ^protochloride of tin, protonitrate of mercury, and the nitrate of siprecipitates (principally metallic rhabarberates and tannates).

Paper coloured by rhubarb is not affected by boracic acid, orborates rendered acid; whereas turmeric paper is reddened b) ,agents (Faraday, Quart. Journ. of Science, vol. vi. p. 152). A “tion of llussian, Dutch-trimmed, or of China rhubarb, becomes, sa solution of iodine, greenish-blue {iodide of starch): after a few im n ^the colour disappears, and no iodine can be detected in the liqu°,j g jjstarch, unless nitric acid be previously added. A decoction of

rhubarb is rendered, by a solution of iodine, intensely blue { l0Ctlstarch), the colour not completely disappearing by standing.

Physiological Effects, [a.) On animals. — On the Solipedes rh gj,acts as a tonic, confining its action principally to the stomach, ' g {digestive power it augments. On the Carnivora it operates, in ® g,shalf a drachm, in the same way; but, in doses of several < ^ raC cfa purgative. On the larger Herbivora it may be given to the exseveral ounces without causing purgation (Moiroud, PharW■ ^ u 0.p. 260). Tiedemann and Gmelin {Versuche ii. d. Wege auf welch■ ^^aus d. Magen u. Darmk. gelang. S. 10-12) detected it by its 3colour in the serum of the blood of the mesenteric, splenic, al . .veins, and in the urine of dogs, to which rhubarb had been admin lSby the mouth. They failed to recognise it in the chyle. . . s a*

( b.) On man.—In small doses (as from four to eight grains) d a j toan astringent tonic, its operation being principally or wholly con* 1 te sthe digestive organs. In relaxed conditions of these parts it l ,r ° 0 f tl* ethe appetite, assists the digestive process, improves the quality g. 0l iialvine secretions, and often restrains diarrhoea. In large dosesa scruple to a drachm) it operates, slowly and mildly, as a P lU ' 1 ’ efl i-sometimes causing slight griping. It never inflames the mucous ^ jJ)ebrane of the alimentary canal, as jalap, scammony, colocynth, an ^.Jiother drastic purgatives are capable of doing. The constipationfollows its cathartic effect has been ascribed to the operation ^ jtastringent matter. In febrile complaints and inflammatory n ise ' £ tli esometimes accelerates the pulse, and raises the temperature g jj 0 ,vbody, whence the impropriety of its use in these cases. I ts ^ g re-colouring matter (rhabarberic acid) becomes absorbed, and may , uCe scognised in the urine, by the yellow stain which this secretion P