(178 THE PRINCIPLES OF SCIENCE. [chap. 



useful or iiseless, abundant or scarce, ferro- magnetic or 

 diamagnetic, and so on. 



Mineralogists have spent a great deal of labour in trying 

 to discover the supposed natural system of classification for 

 minerals. They hav^e constantly encountered the difficulty 

 that the chemical composition does not run together with 

 the crystallographic ibrm, and the various physical pro- 

 perties of the mineral. Substances identical in the foruis 

 of their crystals, especially those belonging to the first or 

 cubical system of crystals, are often found to have no 

 resemblance in chemical composition. The same sub- 

 stance, again, is occasionally found crystallised in tM'o 

 essentially difterent crystallographic forms ; calcium car- 

 bonate, for instance, appearing as calc-spar and arragonite. 

 The simple truth is that if we are unable to discover any 

 correspojidence, or, as we may call it, any correlation between 

 the properties of minerals, we caimot make any one arrange- 

 ment which will enable us to treat all these properties in a 

 single system of classification. We must classify minerals 

 in as many different ways as there are difierent groups of 

 unrelated properties of sufficient importance. Even if, for 

 the purpose of describing minerals successively in a treatise, 

 w'e select one chief system, that, for instance, having regard 

 to chemical composition, we ought mentally to regard the 

 minerals as classified in all other useful modes. 



Exactly the ,same may be said of the classification of 

 plants. An immense number of different modes of classi- 

 fying plants have been proposed at one time or other, an 

 exhaustive account of which will be found in the article on 

 classification in Eees' " Cyclopedia, " or in the introduc- 

 tion to Lindley's " Vegetable Kingdom." There have been 

 the Fructists, such as Cc>3salpinus, Morison, Hermann, 

 Loerhaave or Gaertner, who arranged plants acc(n-ding tn 

 the form of the fruit. The Corollists, Eivinus, Lxidwig, 

 and Tournefort, paid attention chiefly to the number and 

 arrangement of the parts of the corolla. Magnol selected 

 the calyx as the critical part, while Sauvage arranged plants 

 accordino' to their leaves ; nor are these instances more than 

 a small selection from the actual variety of modes of classi- 

 fication which liave been tried. Of such attempts it may 

 be said that eveiy system will probably yield some infoi-- 

 mation concerning the relations of plants, and it is only 



