182 ON THE IMPURITY OP SOME DRUGS. 
bitter principle in a state of greater concentration ; but 
what is the price at which this advantage is obtained ? 
there is only one-third of the quantity of extract. Spirit, 
sp. gr. 0,834, is not a very good vehicle for extracting the 
bitter principle from gentian root ; when it has ceased to 
act, spirit, sp. gr. 0.921, will dissolve a considerable quan- 
tity of the active parts; the latter is, therefore, undoubtedly 
preferable for the preparation of the extract. 
These two examples are sufficient to show the incon- 
venience that may result from adopting an absolute prin- 
ciple, and extending its application. In extracting the 
soluble matter contained in plants, it should not be forgot- 
ten that the solubility peculiar to each of the principles, 
when isolated, is no longer the same when several of these 
principles are associated, and that we must not judge, a 
priori, of the action of solvents upon each of them. 
Ibid, from Ibid. 
ART. XLII1 ON THE IMPURITY OF SOME DRUGS. 
BY DAVID MURDOCH, ESQ. 
1 . Calamine, or Carbonate of Zinc. 
As the calamine, or impure carbonate of zinc, sold in 
London, had been frequently examined by Dr. R. D. 
Thomson, and found always destitute of zinc, it became a 
matter of some interest to ascertain if the same remark ap- 
plied to the calamine which occurs in commerce in Glasgow. 
Accordingly, at the request of Dr. Thomson, a specimen 
was subjected to analysis. The color of this substance is 
well known to be a light red. When it is boiled with mu- 
riatic acid, it effervesces slightly, and becomes perfectly 
