[ 432 ] 



fubtilty of this portion of the drug in pervading the finer 

 feries of vefTels ; and its retaining this tinging property fo 

 tenacioufly in Experiment IX. after a tincture had been 

 drawn from it by ardent fpirits, nay, even after circulating 

 with various animal fluids, feems to point out, that, were 

 it not for the high price it bears, rhubarb might be turned 

 to good account as a colouring fubftance. Its yellow 

 colour refills aquafortis, and other acids, which deftroy 

 the colour of infufions of faflfron, turmerick, and other ve- 

 getable yellows. 



From this experiment wc further learn, that the tinging 

 property is diftinft from the aftringent quality, fince the 

 urine, though deeply tinged with it, communicated no 

 vifible blacknefs to a folution of martial vitriol, which it 

 would not have failed to have done, had the urine con- 

 tained any aftringent impregnation. The aftringent qua- 

 lity of this drug, therefore, does not appear to enter the 

 blood vefTels, agreeably to what I have elfewhere remarked. 

 concerning the Peruvian bark.* 



From Experiment VIII. it appears, that the two foreign 

 rhubarbs yield more foluble parts to proof fpirit, than either 

 of the Englifh, and that of the latter No. II. rather fur- 

 pafTes No. I. in this refpe£l. But it is evident from the 

 fubfequent experiments, that the purgative power of rhu- 

 barb does not keep pace with its folubility in ardent fpirits, 

 "becaufe the refiduum, which remains infoluble in that men- 

 ftruum, yet retains the purgative virtue; and the Eafl- 

 Indian, though more foluble, is yet lefs purgative than the 

 Englifh. It is remarkable, that the fpirituous extract of 

 rhubarb does not purge, but that the extract made by 

 water after fpirit does ; as if the purgative quality refided 

 chiefly in a combination of gummy and faline matter. 



* In my fecood letter to Dr. Savkdkrs, on the redPeruvian bark. 



The 



