810 MR HAIDINGER on the Determination of the Species in 



viduals, whose natural-historical properties, that is to say, those 

 properties which may be observed while the mineral continues 

 to exist, are either absolutely the same, or present gradations 

 which form continuous series. These are either determinable a 

 priori, from one given member, or they are the result of accurate 

 and long continued observation. In the latter we are not per- 

 mitted to venture one step beyond the boundaries of actual ob- 

 servation, while, in regard to form, we are entitled to suppose, 

 according to analogy, that, whatever may be wanting for the 

 necessary completion of the representation, may still be supplied 

 in establishing the idea of the species. 



We must not pass over unnoticed any of the characters or 

 natural-historical properties, otherwise the idea of the species it- 

 self would be incomplete ; the variety displayed in nature could 

 not be explained sufficiently, nor could it be demonstrated that 

 we are really entitled to consider certain bodies, under the required 

 circumstances, as belonging to one and the same species, although 

 they differ essentially in some of their properties. The properties 

 which are most useful, and must be particularly attended to, are 

 those which can be expressed by numbers or measurement ; in 

 fact, those which allow of the application of mathematics. The 

 properties referring to form and cleavage, simple and double re- 

 fraction, angle of maximum polarisation, and the dispersive power, 

 hardness and specific gravity, are therefore most minutely to be 

 examined, if we wish to arrive at a correct result. But it would 

 be contrary to the principles of natural history to determine the 

 idea of the species according to one single property, because, by 

 being thus confined, we would soon fancy ourselves under the 

 necessity of introducing considerations which are foreign to the 



science *. 



* C'est dans ces sortes de cas ou les caracteres geometriques se taisent, qu'il de- 

 vient necessaire d'avoir recours a la Chimie. HAOY, Traits, 2de Ed. t. iv. p. 179- 



