18 ON THE DIOPSIDE. 



from the angle determined by observation. This value is 

 too tar from the truth, not to lead us to a suspicion of some 

 irregularities in the crystal, which that celebrated crystallo- 

 Geometrical grapher examined. It is equally certain, that calculation 

 calculation would have acquainted him with his mistake, in affording 

 cessary him a sure method of correcting his observation. I might, 



mention other instances, if T were not afraid of wandering 

 from my subject. That which I have adduced is sufficient, 

 to show the justice of the remarks on the manner of describ- 

 ing crystals may by Mr. Hatiy in the Annals of the Museum 

 assisted by the f Natural History. This gentleman has there shown, that 

 ligation. " tn e descriptions of a crystal, to be precise, must indicate the 

 angles as determined by the concurrence of common geo- 

 metry with that founded on the structure of minerals. By 

 following this method, the only one compatible with strict 

 accuracy, we shall be certain, that the angles will always 

 agree with each other. They will be as so many limits, to* 

 which the observer may come sufficiently near with the goni- 

 ometer, to be able to refer a crystal to the species or variety, 

 the characters of which it bears. This is all we can expect 

 from this instrument, however nicely it may be executed, 

 and however skilful the hand that employs it. 

 The diopside To return to the diopside. Though this substance has 

 a distinct spe- never ve t been subjected to analysis, Mr. Hatiy does not he- 

 sitate to consider it as a species, that should occupy a dis- 

 tinct place in the system*. The few characters I have given 

 are sufficient, to demonstrate this assertion, because they 

 occupy the first rank among those that are truly specific. 

 The primitive form obtained by mechanical division differs 

 from all other known forms. Far from having the character 

 of a limitation, it is remarkable for a singularity, that no 

 other species has yet presented. This consists in the double 

 appearance it exhibits, one in its prism, which is a foursided 



The diopside • The place assigned to the diopside is immediately aft«r the pyrox- 

 ilf-!., 5 ?!^* 1 ?!^ ene, the primitive form of which bears some analogy to it. In each the 

 primitive form is a quadrangular prism : but Mr. Haiiy has ascertained, 

 that this prism is rectangular in the diopside, while the faces of that of the 

 pyroxene are inclined to each other in angles of about 92° and 88°. Be- 

 sides, the primitive form of the diopside is subdivisible in the direction of 

 the two diagonals of its bases, while that of the pyroxene is capable of 

 fceing subdivided enly parallel to the greater diagonal. 



rectangle^ 



logy to thepy 

 xoxeue 



