. 208. PHYSICAL PROPERTIES OF MINERALS. 301 



ness. It is necessary therefore to endeavour, even without 

 an accurate scale of that kind, to become capable of ascer- 

 taining and indicating these differences, at least with a de- 

 gree of accuracy and certainty sufficient for the wants of 

 the Natural History of the Mineral Kingdom. 



The existence of differences in the degrees of hardness 

 among the minerals, is very easily discovered, by the 

 simple experiment of scratching one of them by the other. 

 A sharp corner of rhombohedral Quartz will produce a deep 

 cut in the mass of rhombohedral Lime-haloide ; whilst a 

 sharp corner of the latter species does not injure the sur- 

 face of the former. Hence we infer, that rhombohedral 

 Quartz possesses a higher degree of hardness than rhom- 

 bohedral Lime-haloide ; and in general, that of two mine- 

 rals, the harder one scratches the other, but cannot inversely 

 be scratched by it. Some precautions, however, are neces- 

 sary in drawing general inferences from these observations. 



If we proceed upon this principle, we may obtain a Scale 

 for the degrees of hardness, answering in every respect the 

 purposes of Mineralogy. This is effected by choosing a 

 certain number of suitable minerals, of which every pre- 

 ceding one is scratched by that which follows it, while the 

 latter does not scratch the former ; taking care always that 

 the intervals between every two members of the scale be 

 not so disproportionate, as either to render its employment 

 more difficult, or to hinder it altogether. 



The following scale possesses these properties : 



1, Prismatic Talc-mica, the common Talc of mineralogists, 

 of a whitish or greenish colour. 



2, Prismatoidal Gypsum-haloidc, a variety imperfectly 

 cleavable, of an inferior degree of transparency, and not 

 crystallised; crystals and perfectly transparent varieties 

 being rather too soft. This degree of hardness is exactly 

 that of hcxalicdral Rock-salt, which mineral therefore may 

 be very useful, either in being immediately employed in 

 the determination of hardness, or at least in finding out 

 such varieties of the above-mentioned species, as exactly 

 possess the required degree of hardness. 



