228 HISTORY OF MINERALOGY. 



rized test of the difference of substances which 

 nearly resemble each other. 



Enough has been said, probably, to enable the 

 reader to judge how much each of the two persons, 

 now under review, contributed to crystallography. 

 It would be unwise to compare such contributions 

 to science with the great discoveries of astronomy 

 and chemistry; and we have seen how nearly the 

 predecessors of Rome and Haiiy had reached the 

 point of knowledge on which these two crystallo- 

 graphers took their stand. But yet it is impossible 

 not to allow, that in these discoveries, which thus 

 gave form and substance to the science of crystal- 

 lography, we have a manifestation of no common 

 sagacity and skill. Here, as in other discoveries, 

 were required ideas and facts ; clearness of geo- 

 metrical conception which could deal with the most 

 complex relations of form ; a minute and extensive 

 acquaintance with actual crystal ; and the talent 

 and habit of referring these facts to the general 

 ideas. Haliy, in particular, was happily endowed 

 for his task. Without being a great mathematician, 

 he was sufficiently a geometer to solve all the pro- 

 blems which his undertaking demanded ; and though 

 the mathematical reasoning might have been made 

 more compendious, by one who was more at home 

 in mathematical generalization, probably this could 

 hardly have been done without making the subject 

 less accessible and less attractive to persons mode- 



