On tht Operation of Medicines, 203 



endued with equal fenfibility, it is entirely inert. 

 To what property in opium, capable of affefting 

 the external fenfes, are we to afcribe its narcotic 

 powers ? Or is there in the grateful tafte of 

 faccharum Jaturni any indication of a deadly poi- 

 fon ? But the inftances are numberlefs which 

 may be adduced to prove the uncertainty of rea- 

 foning otherwife than from obfervation, con- 

 cerning the aftion of medicines, and the peculiar 

 fenfibility of different parts of our fyftem to their 

 impreflion. Following, therefore, experience 

 as our guide, let us notice fuch facls as may 

 elucidate the fubjeft before us. It is well known 

 that madder root carries its tinging quality to 

 the bones, affefting neither the fkin, the muf- 

 cles, the ligaments, nor fat. Digeftion confe- 

 quently leaves this tinging quality unchangfd ; 

 or perhaps it is again recovered, when arrived 

 at the bones, by fome new arrangement of parrs 

 produced by the chemiftry of nature. Extraft 

 of logwood, taken internally, fometimes gives a 

 bloody hue to the urine. But the aftringency 

 of it does not, according to my trials, accom- 

 pany Its colouring matter*. I recoUedt no in- 



ftancc 



• It is faid the fruit of the Nopal, or Indian fig, on which 

 the cochineal is propagated, tinges the urine of thofe who 

 eat it with a deep blood colour. The leaves of this fhrub 

 are of a permanent and lively green ; and it is remarkable 

 tjiat their juices are converted, by the concodllve organs 



of 



