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XIV. On the Molecular' Constitution of Crystals. 

 By Robert T. ForsteRj A.B."^ 



[With Two Plates.] 



ANY theories which have been advanced to account for the 

 phsenoniena of crystalHzation have been but short Uved in 

 their reputation ; and, indeed, it may be safely doubted if any 

 occupy such a position as to entitle them to much credit. 



The subject is itself one of much interest ; and although the 

 consideration that so many able minds have failed in the attempt 

 to investigate it may naturally deter an inquirer, still the prize 

 is sufficient to persuade him to hazard the attempt. 



I shall first take a short review of tlie many theories which 

 have been advanced in explanation of the phjenomena, and I 

 trust I shall be able to show that some were unquestionably 

 faulty, and all undoubtedly and confessedly deficient. 



Passiug by the many absurd theories advanced on this subject 

 in ancient times, the first whom we find deserving of notice is 

 Christian Huygens. lu his celebrated work on double refrac- 

 tion, he considered the crystals of Iceland spar to be built up of 

 spheroids, which by their unequal density separated the incident 

 light into two rays. He did not, however, give any reason why 

 the particles are so aggregated. 



Hooke, in his Micrographia, advanced a similar hypothesis, 

 except that he considered the atoms to be spherical ; a supposi- 

 tion which would have accounted for forms in the first system, 

 but which would have utterly failed in case of the third or 

 " rhombohedral," to which he applied it ; nor does he perceive 

 that the molecules, if left to themselves, would not assume a 

 definite arrangement. He does not seem to consider these 

 spheres as the ultimate atoms; he says, " that having already 

 shown how a fluid will naturally assvmie the spherical form, he 

 will proceed to show how these spheres will unite to form a 

 crystal." On the absurdity and complication of such ideas I 

 need not dwell. 



The next who commanded attention was M. Prechtl de Brun, 

 whose ideas were to a certain extent those of Hooke ; he con- 

 sidered a fluid to be made up of soft molecules, which, while the 

 body was undergoing its change of state, suffered a change of 

 form arising from their mutual pressure, and that they were 

 thus formed into cubes, dodecahedrons, &c. ; the cubes by their 

 union forming larger cubes, &c. Not only was this hypothesis 

 quite insufficient, inasmuch as it substituted one difficulty for 



* Communicated by the Author ; liaving been read before the Royal 

 Irish Academy, May 14, 1855. 



