CLASSIFICATION. 349 



is to be selected, to the exclusion of all others. This 

 erroneous notion probably proceeds also in part from the 

 limited powers of thought and the inconvenient mechani- 

 cal conditions under which we labour. If we arrange the 

 books in a library catalogue, we must arrange them in 

 some one order ; if we compose a treatise on mineralogy, 

 the minerals must be successively described in some one 

 arrangement ; if we describe even such simple things as 

 geometrical figures, they must be taken in some fixed 

 order. We shall naturally therefore select that classification 

 which appears to be most convenient and instructive for 

 our principal purpose. But it does not follow that this 

 system of classification possesses any exclusive excellence, 

 and there will be usually many other possible arrange- 

 ments, each valuable in its own way. A perfect intel- 

 lect would not confine itself to one order of thought, 

 but would simultaneously regard a group of objects as 

 classified in all the ways of which they are capable. 

 Thus the elements may be classified according to their 

 atomicity into the groups of Monads, Dyads, Triads, 

 Tetrads, Pentads, and Hexads, and this is probably the 

 most instructive classification ; but it does not prevent 

 us from also classifying them according as they are 

 metallic or non-metallic, solid, liquid or gaseous at ordi- 

 nary temperatures, useful or useless, abundant or scarce, 

 ferro-magnetic or diamagnetic, and so on. 



Mineralogists have spent a great deal of labour in 

 trying to discover a so-called natural system of classifi- 

 cation for minerals. They have constantly encountered 

 the difficulty that the chemical composition did not 

 run together with the crystallographic form, and the 

 various physical properties of the mineral. Substances 

 identical in the form of their crystals, especially those 

 belonging to the first or cubica] system of crystals, were 

 often found to have 110 resemblance in chemical compo- 



