. 
Vi PREFACE. 
from the peculiarity of their powers, or the facili- 
ty of their acquisition, they are calculated to take 
the place of others previously in use. 
Of our present stock of medicinal agents, col- 
lected from various parts of the globe, a few ap- 
pear to be unique in their powers, and could not 
in the present state of our knowledge, be super- 
seded by other substances. A number more pos- 
sess active properties, yet of a kind, for which sub- 
stitutes might be found among the native produc- 
tions of almost every country into which they are 
imported. There are others which possess little 
activity or value, but which, froma sort of fashion, 
are still articles of commerce and consumption. 
In the management of diseases, the physician 
requires instruments of determinate power, on the 
operation of which, he may build definite expec- 
tations. Many such are already in his hands, 
Yet when we consider how small a portion of the 
vegetable kingdom has been medically examined, 
there can be little doubt that a vast number 
of active substances, many perhaps of specific effi- 
cacy, remain for future inquirers to discover. 
In this respect, every successive age is making 
acquisitions. But a century or two ago, the eiy- 
ilized world were unacquainted with the proper- 
ties of ipecacuanha, of jalap, and the Peruvian 
