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CHAPTER II. 



EPOCH OF ROME DE LISLE AND HAUY. ESTABLISH- 

 . MENT OF THE FlXITY OF CRYSTALLINE ANGLES, 

 AND THE SIMPLICITY OF THE LAWS OF DERIVA- 

 TION. 



TTTE have already seen that, before 1780, several 

 f V mineralogists had recognized the constancy of 

 the angles of crystals, and had seen (as Demeste and 

 Werner,) that the forms were subject to modifica- 

 tions of a definite kind. But neither of these two 

 thoughts was so apprehended and so developed, as to 

 supersede the occasion for a discoverer who should 

 put forward these principles as what they really 

 were, the materials of a new and complete science. 

 The merit of this step belongs jointly to Rome de 

 Lisle and to Haiiy. The former of these two men 

 had already, in 1772, published an Essai de Cristal- 

 lographie, in which he had described a number of 

 crystals. But in this work his views are still rude 

 and vague ; he does not establish any connected 

 sequence of transitions in each kind of substance, 

 and lays little or no stress on the angles. But in 

 1783, his ideas 1 had reached a maturity which, by 

 comparison, excites our admiration. In this he as- 

 serts, in the most distinct manner, the invariability 



1 Crislallographie, ou Description de Formes propres a tons 

 les Corps du Regne Mineral. 3 vols. and 1 vol. of plates. 



