as a Branch of Natural History , 4*c. 243 



§ S5, The orders will be formed in some classes of genera 

 having the same electro-negative element, whether its presence 

 and particular nature be indicated by a chemical character, and 

 especially by the blowpipe, the most ready and simple test for 

 such elements, or when such mineralogical character may not be 

 found, by an assemblage of characteristic, physical, or external 

 properties. In other classes, where a greater resemblance pre- 

 vails among the genera, a common important chemical property 

 and mineralogical character, will be the tie which will bind toge- 

 ther in an order many families, having each a different electro- 

 negative element. 



The existence of isomorphism, though much less frequent be- 

 tween acids or electro-negative, than between bases or electro- 

 positive, elements, will sometimes require the establishment of 

 suborders, as well as the absence or presence of water of com- 

 position in the chemical combination. The blowpipe affords an 

 excellent character to ascertain this last fact, and the hydrated 

 substances offer generally very essential distinctive characters in 

 their inferior hardness, and specific gravity, and in their easier 

 frangibility, as well as in their form, from substances composed 

 of the same elements, but anhydrous. 



§ 6Q. It remains for us to find for the highest divisions or 

 classes, some great and general characters, comprehending whole 

 groups of orders, which agree at the same time in their general 

 composition, and in some common and distinctive physical pro- 

 perties. We must remember that here, as well as in the other 

 lower divisions, it ought to be positive characters immediately 

 belonging to the individuals themselves, and not properties of 

 one or other of their constituent parts or elements, or abstract 

 notions of the nature of their chemical composition, as has been 

 hitherto done. 



Now, we find that a great part of the orders are composed of 

 individuals chemically formed of combustible or inflammable 

 substances, either alone or combined with each other, and con- 

 taining none of those substances by which combustion is sup- 

 ported, such as oxygen, fluorine, and chlorine, and that by far 

 the greater number of genera of this description (69 out of 77, 

 or nearly /^ths) agree together in possessing a complete metallic 



e2 



