278 TERMINOLOGY. . 196. 



application. The properties most important, in Deter- 

 minative Mineralogy (. 11.) are the different kinds of 

 lustre, the metallic colours, and particularly the phe- 

 nomena of simple and double refraction, as observed in 

 common and polarised light. As yet we cannot dispense 

 in the characters with the first and the second, although 

 they are of comparatively less value. The third are 

 of great consequence : in many cases they are sufficient 

 for determining the system of crystallisation, in such 

 varieties as do not present any regular form, or any 

 traces of cleavage ; they very often yield neat and de- 

 cisive characters, where the minerals are much resem* 

 bling each other in the rest of their properties, and 

 they are most useful in ascertaining whether a mineral is 

 simple or compound. For the Descriptive Mineralogy 

 (. 11.) all the optical properties of the minerals are of 

 equal consequence, and at the same time not inferior to 

 any other properties. These descriptions have not the pur- 

 pose of distinguishing objects, but that of producing an 

 image of them ; and to this the colours confer no less than 

 the forms, the knowledge of lustre and transparency no 

 less than the degrees of hardness or of specific gravity. 



The optical properties must therefore by no means be ne- 

 glected, although many of them are of less influence than 

 those deriving from the forms to the scientific progress of 

 Mineralogy. Very often by their assistance we may dis- 

 pense with the use of the Characteristic, because they are 

 very well calculated for recalling to our mind such va- 

 rieties of the same species as we have already determin- 

 ed. They are obvious at first sight, and therefore easily 

 observed and determined under whatever circumstances 

 they may be found, they are in particular recommended to 

 those who wish to acquire an extensive general knowledge 

 of the productions of the mineral kingdom. 



. 196. KIND. AND INTENSITY OF LUSTBE. 



The lustre of minerals is considered in respect 

 to its Kmdy and in respect to its Intensity. 



