104 TERMINOLOGY. 



These subjects are treated of in mineralogy, only so far as they al- 

 low of some application in discriminating and describing minerals. 

 In order to employ them to any purpose, it is necessary to deter- 

 mine and to provide with peculiar denominations, those differences 

 which may be distinguished in these properties, both in respect to 

 their kind and intensity. This will require us to fix a certain im- 

 pression upon our mind, and always to designate this impression 

 with the same name, so as to recall it to our memory, whenever we 

 read this name, or hear it uttered. It is necessary therefore to have 

 experienced these impressions upon our own mind, since explana- 

 tions cannot be substituted in their place. An acquaintance with 

 these properties may-be acquired from the consideration of bodies, 

 which are not minerals. But it is best and most easily obtained by 

 addressing ourselves to the bodies themselves. The pupil is there- 

 fore advised to familiarize his mind with the distinctions detailed un- 

 der the present characters, by an attentive examination of a col- 

 lection of specimens arranged expressly for their illustration. 



In the determination of species, the present characters are rarely 

 of much value; but in descriptive mineralogy, where the business 

 is not to distinguish objects, but to produce an image of them, the 

 optical properties of minerals are not inferior in importance to any 

 other properties. 



The optical characters must therefore, hyno means, be neglected, 

 although many of them are of less importance than those derived 

 from the forms to the progress of mineralogy. Very often by their 

 assistance, the pupil may dispense with the use of the characteristic, 

 1 because they are very well calculated for recalling to his mind such 

 varieties of the same species as he has before determined. They are 

 obvious at first sight, and are capable of being observed and deter- 

 mined under whatever circumstances they may be found. 



. 88. KINDS AND INTENSITY OF LUSTRE, 



The lustre of minerals is considered in respect to its kind, 

 and in respect to its intensity. 



The kinds of lustre are : 



1. Metallic lustre, 



2. Adamantine lustre, 



3. Resinous lustre, 



4. Vitreous lustre, 



5. Pearly lustre. 



