CHARACTERISTIC. 149 



for the distinction aimed at; since it so happens that in sev- 

 eral of the orders, and especially in the first order of the third class, 

 a number of species frequently coalesce so far as this property is 

 concerned, or form equal members in the series. In order for a dis- 

 tinction in these cases, additional marks require to be added. Ac- 

 cordingly, the specific gravity, sometimes the angle at which par- 

 ticular faces incline, (when the mineral is crystallized or semi- 

 crystallized,) occasionally also the lustre, streak, taste, &c. are added 

 to complete the character. In the second and third orders of the 

 third class, the property according to which the species are arrang- 

 ed is specific gravity ; and this, of course, furnishes the first term of 

 the character among these species. 



It is obvious, that in distinguishing the species by characters, we 

 need more marks than are really indispensable merely to exclude 

 an individual from those species to which it does not belong ; for we 

 wish to be assured that it does belong to the species under which 

 our arrangement of characters finally brings it. For example, in. 

 the first order of the first class, the two species Horn Silver and 

 Common Salt are capable of berng distinguished from each other, 

 merely by the property of hardness, that of the first being = 1... 1-5, 

 and that of the second being = 2. But, another vineral maybe 

 found which on account of its properties comes within this order, and 

 which is possessed of the hardness of one or the other of these spe- 

 cies. Under these circumstances, provided the characters of Horn 

 Silver and Common Salt are confined to the property of hardness, 

 this mineral, though really distinct, must coalesce with the one with 

 which it agrees in its character ; whereas had the specific characters 

 of these species been extended so as to include the property of spe- 

 cific gravity, it might have easily been discovered that it was a dis- 

 tinct species. It is on this account, therefore, that the characters of 

 the species never consist of less than two marks, and sometimes they 

 are possessed of three or even four. 



. 122. USE OF THE CHARACTERISTIC. 



The use of the characteristic in mineralogy is the same 

 as in zoology and in botany. 



If a mineral is to be determined, the first step relates to its form. 

 If regular, the system of crystallization requires to be ascertained. 

 If irregular, but yielding a regular form by cleavage, the same 



13* 



