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PREFACE 
              jT THE SECOND VOLUME. 
    As frequent use is made in these pages of 
observations drawn from the auxiliary sciences, as 
affording some light on the medicinal properties of 
plants, it may be proper to examine how far tes- 
timony of this kind is entitled to receive credit in 
our inquiries and examinations. 
    There can be no question, that the actual op. 
eration of medicines upon the human system, 
gathered from positive experience, is, in the pres- 
ent state of our knowledge, the only criterion by 
which we can pronounce, with universal certainty, 
on their properties.   There are nevertheless 
many things to be learnt from chemical analysis, 
sensible qualities, and botanical affinity, which 
may afford us, in some instances certainty, and" in 
most others presumptive evidence of the medicinal 
characters of vegetables. The correspondence 
in these respects is frequently so striking, that 
we can hardly resist the belief, that an entire har- 
mony of properties exists, which, if we are unable 
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