CLASSIFICATION. 131 



Cannot be removed or made to disappear by the idea of a series, and 

 accordingly tbe individual cannot be brought under the idea of 

 identity. 



The same series, as respects lustre and color, have been seen to 

 exist, (. 89,93.) : with regard to specific gravity and hardness also, 

 a similar gradation is admitted, since the determination of these 

 properties in individuals of the same species does not admit of rig- 

 orous limits.* In these cases, it is to be recollected that the series 

 arises out of an uninterrupted connexion among a number of indi- 

 viduals, capable of being so arranged that any two will pass insensi- 

 bly into each other, and that the chain no where presents any in- 

 terruption or want of continuity. In the comparison of individuals, 

 this series may take place in but one of the above mentioned prop- 

 erties, while in the remaining ones there exists the most perfect 

 identity, or it may take place in two or three, or all of them, at the 

 same time. To illustrate the idea of identity by series in the char- 

 acter of hardness, for example, we will suppose the comparison of 

 four individuals of the species Beryl. Let them all agKjee, as res- 

 pects form, color, lustre and specific gravity ; but in hardness we 

 find them to be represented by 7.5, 7.6, 7.7 and 7.8. They are iden- 

 tical, for thougk differing in hardness, yet the differences are mem- 

 bers of a continuous series. If, however, an individual of the spe- 

 cies Apatite be compared with these individuals, the form, color and 

 lustre is exactly similar in both cases, but the hardness of the crys- 

 tal of Apatite we shall find to be 5, a discovery which immediately 

 destroys the idea of identity, since it is obvious that the difference 

 between 5 and 7.5 is far greater than that between 7.5 and 7.8, 

 which are the extremes of the series in the hardness of the indi- 

 viduals of Beryl. They cannot therefore be said to be identical, 

 since the numbers expressive of their hardness do not form a con- 

 tinuous series. 



By the above process, extended to all those properties which form 

 series by these gradations, w r e may include the assemblage of all 

 those individuals, which, notwithstanding their differences, may yet 

 be brought under the idea of identity. At the same time, those in- 

 dividuals which do not allow the process to be applied to them, are 



* The series, as respects the properties of hardness and specific gravi- 

 ty, no doubt arises, in a majority of instances, from the intermixture of 

 foreign minerals among the individuals of a species, in addition to the im- 

 perfection of our instruments for obtaining accurate results. 



