PREFACE'. ■ 
weapons of defence against the depredations of 
a variety of animals, which would otherwise 
frustrate some higher intention. 
Many of these plants, by the long experience 
of mankind, have already been converted into 
medicine, and other useful purposes; and by 
the diffusion of botanical science, it is hoped, 
in many others, the virtues which yet lie dor¬ 
mant will be awakened; and that those plants 
which are now ranked amongst destructive 
poisons, will gradually be reclaimed, and be¬ 
come a valuable acquisition to the science of 
medicine. 
The analogy of plants as to their virtues, is 
well worth the enquiry of some able botanist, 
as the same virtues which are observed in a 
genus, do in a great measure run through all 
the species ; and in some cases a whole order, 
and even a whole class, if natural, will have the 
same predominant virtue.* 
* Tournefort had a very easy method to discover the acid or alka¬ 
line quality of plants ; he made use of a deep blue paper, which 
being moistened with the juice of the .plant, shewed its quality ; with 
an acid it becomes red, with an alkali, green.-—A vegetable blue will 
