EPOCH OF DE LISLE AND HALT. 



i)oked upon a? a great discoverer, if his fame had rm t been dimmed 

 by the more brilliant success of his contemporary Haiiy. 



Bene-Just Haiiy. is rightly looked upon as the founder of the 

 modern school of crystallography ; for all those who have, since him r 

 pursued the study with success, have taken his views for their basis. 

 Besides publishing a system of crystallography and of mineralogy, far 

 more complete than any which had yet appeared, the peculiar steps in 

 the advance which belong to him are, the discovery of the importance 

 of cleavage, and the consequent expression of the laws of derivation 

 of secondary from primary forms, by means of the decrements of the 

 successive layers of integrant molecules. 



The latter of these discoveries had already been, in some measure, 

 anticipated by Bergman, who had, in 1773, conceived a hexagonal 

 prism to be built up by the juxtaposition of solid rhombs on the 

 planes of a rhombic nucleus. 4 It is not clear 5 whether Hatiy was 

 acquainted with Bergman's Memoir, at the time when the cleav- 

 age of a hexagonal prism of calcspar, accidentally obtained, led him 

 to the same conception of its structure. But however this might be, 

 he had the indisputable credit of following out this conception with 

 all the vigor of originality, and with the most laborious and persever- 

 ing earnestness ; indeed he made it the business of his life. The 

 hypothesis of a solid, built up of small solids, had this peculiar advan- 

 tage in reference to crystallography ; it rendered a reason of this 

 curious fact ; that a certain series of forms occur in crystals of the 

 same kind, while other forms, apparently intermediate between those 

 which actually occur, are rigorously excluded. The doctrine of decre- 

 ments explained this ; for by placing a number of regularly-decreasing 

 rows of equal solids, as, for instance, of bricks, upon one another, we 

 might form a regular equal-sided triangle, as the gable of a house ; 

 and if the breadth of the gable were one hundred bricks, the height 

 of the triangle might be one hundred, or fifty, or twenty-five ; but it 

 would be found that if the height were an intermediate number, as 

 fifty-seven, or forty-three, the edge of the wall would become irregu- 

 lar; and such irregularity is assumed to be inadmissible in the regular 

 structure of crystals. Thus this mode of conceiving crystals allows of 

 certain definite secondary forms, and no others. 



The mathematical deduction of the dimensions and proportions 



4 De Formis Crystallorum. Nov. Act. Reg. Soc. Sc. Ups. 17 7 o. 

 * Traite de Miner 1822, i. 15. 

 VOL. II. 21 



