398 THE PRINCIPLES OF SCIENCE. 



of objects into two distinct parts, and each part may be 

 sub-divided successively by any prominent and well 

 marked circumstance which is present in a large part of 

 the genus and not in the other. To refer an object to its 

 proper place in such an arrangement we have only to note 

 whether it does or does not possess the successsive critical 

 circumstances. Dana devised a classification of this kind z 

 by which to refer any crystal to its place in the series of 

 six or seven classes already described. If a crystal has all 

 its edges modified alike or the angles replaced by three or 

 six similar planes, it belongs to the monometric system ; 

 if not, we observe whether the number of similar planes 

 at the extremity of the crystal is three or some multiple 

 of three, in which case it is a crystal of the hexagonal 

 system ; and so we proceed with further successive dis- 

 criminations. 



To ascertain the name of a mineral by examination with 

 the blow-pipe, an arrangement more or less evidently on 

 the bifurcate plan, has been laid down by Von Kobell a . 

 Minerals are divided according as they possess or do not 

 possess metallic lustre ; as they are fusible (including 

 under fusible substances those which are volatile) or not 

 fusible in a determinate degree, according as they do or 

 do not on charcoal give a metallic bead, and so on. 



Perhaps the best example to be found of any arrange- 

 ment simply devised for the purpose of diagnosis, is 

 Mr. George Bentham's ' Analytical Key to the Natural 

 Orders and Anamolous Genera of the British Flora,' 

 given in his ' Handbook of the British Flora V In this 



z Dana's ' Mineralogy,' vol. i. p. 123. Quoted in "Watts's ' Dictionary of 

 Chemistry/ vol. ii. p. 166. 



a ' Instructions for the Discrimination of Minerals by Simple Chemical 

 Experiments,' by Franz von Kobell, translated from the German by R. C. 

 Campbell, Glasgow, 1841. 



b Edition of 1 866, p. Ixiii. 



