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 



o 



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 



