THIRD PERIOD. 11$ 



stant forms of crystals; but they had presented 

 only detached facts, of which M. Hauy divined 

 the cause, and by the aid of geometry attained 

 the general results, which have changed the basis 

 of the science. Having demonstrated the prin- 

 ciples of the discovery, he applied it in his treatise 

 of mineralogy, published in 1800. This work, 

 by fixing the classification of minerals in charac- 

 ters depending on the nature of their primitive 

 molecules, gave an impulse to the science, similar 

 to that which the discoveries of Lavoisier had 

 given to chemistry, and in the same manner sub- 

 jected it to an uniform course and a regular no- 

 menclature ; but with this difference, that the 

 theory and nomenclature of Lavoisier are modi- 

 fied by successive experiments, while the laws of 

 crystalization are invariable, rigorously deter- 

 mined, and certain in their application from the 

 measurement of the angles. M. Haiiy was called, 

 on the 1 8th of December 1801, to fill a chair for 

 which there could be no competition ; and from 

 that time the instruction has been conformed to 

 the crystallographic method. 



It was at first feared, that this method would 

 embarrass students not prepared to understand 

 it; but M. Haiiy found means to smooth its aspe- 

 rities, and to render sensible the laws of decre- 

 ment and transformation, by models; while, by 



8. 



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