PREFACE. 
XI 
The other plan would be much more comprehensive. 
It would comprise so much of the labours of pre~ 
ceding writers, as should serve to furnish a correct 
history of the progress of knowledge respecting each 
subject of examination. Their several opinions would 
be noticed, and their agreement or disagreement with 
the more accurate knowledge of the nature of these 
substances, which later observations have furnished, 
would be also stated, A sedulous examination and 
careful comparison of different specimens of the same 
species would be required for this purpose ; and, where 
indisputable inferences could not thus be obtained, such 
conjectures might be offered as analogy and probability 
appeared to support. Most of the subjects for the plates, 
which would accompany a work conducted on this 
plan, would be necessarily original ; by which the gene- 
ral fund of information, respecting these much neglected 
subjects of admiration, would be increased : a circum- 
stance much to be desired ; since, in even the most re- 
spectable works which have appeared, the figures have 
been, too frequently, merely copies from those of former 
works. 
The superior advantages which would thus be pro- 
duced were so obvious as to lead to the determination 
of attempting to form a work upon this model. It was 
not however overlooked, that considerable difficulties 
