34» MINERALOOICAL sVStEM^. 



Enquiries on thefirfi Fad;. 

 On the analyfis I. All cryHals that can be fplit by means of inftruraents, of- 

 of cryftals into ^^^ *® ^^^® vIew, if fplit in certain dire6tions, plane and fmooth 

 regularly formed furfaces. If divided in other diredlions, the fra6lureis rugged, 

 fiffurt? ''^ ^ "^"^ ^'^^ word fplit, and notfatced or cut, as the feaions of the 



cryftal are not to be obtained by flow and continued efforts, but 

 by fudden fliocks. Patience, dexterity, and habit enabled the 

 Abb^ to fplit a great number of cryftals ; in all he difcovered 

 plane fmooth furfaces when fplit in certain dire6lions, but when 

 in other diredions, the fra6lure was always rugged and irregu- 

 lar. I requeft. Sir, your attention to this important fa<5l, it is 

 fundamental, and the more important, as feveral perfons of 

 much general information have neglected to attend to it, and, 

 It'iaexferttnentaij in confequence, have fuppofed the whole of this theory to be 

 troncations*of g^ountied on hypothefis. It would be equally erroneous 10 

 De rifle were confound thefe fed^ions of cryftals with De L'Ifle's troncations. 

 bjpotheticaU 'p|^g latter indeed warns his readers, that by the word tronca- 

 tions he wiflies only to figure the appearance of the cryftal ex- 

 amined. They are not therefore real, but only a means of 

 warping the imagination to the exterior form of the cryftal, and 

 are by their nature only defcriptive. The Abb^ Hauy's fee- 

 lions are real, and are pointed out to the obferver by the inte- 

 rior ftrudure of the cryftals ; they are experimental. 

 The inclinations 2. The plane fmooth furfaces obtained by the above method 

 cLftanu" ^* are refpedively parallel to 3, 4 or 6 planes. The mutual in- 

 clination of thefe planes to each other are conftant in cryftals 

 of the fame fubftance, whatever may be the exterior form of 

 the cryftal. Native antimony, phofphate of lead, and quartz 

 feem to fliew an appearance of more than fix planes, and the 

 Ahh€ Hauy leans to the opinion of only five planes in forae 

 cafes ; but as thefe are exceptions to the general rule, and 

 would only tend to complicate this ftatement, I fhall take no 

 further notice of them. 

 Explanation of Let US fuppofe the fmooth furfaces to be only parallel to three 

 the figures and pja^^eg^ or in other words, that the fabftance will only fplit in 

 cryftalsTy^the ^^^^^^ diredions, in that cafe the feclions can only produce a 

 meibodof Hauy. parallelipipedon, whofe nature is determined by the mutual in- 

 clination of the planes to each other. If the planes are per- 

 pendicular, it will be redangular, &c. 



We next fuppofe the fmooth furfaces to be parallel to four 



planes. Here adiftindtion arifes, whether three of thefe planes 



4 have 



