217 ) 



. 



XV. On the Forms of Crystallisation of the Mineral called the 

 Sulphato-tri-Carbonate of Lead. By W, HAIDINGER, 

 Esq. F. R. S, E. 



'(Read February 16. 18H.) 



JL HE history of the progress of discovery in the Natural Sci- 

 ences, records innumerable instances, where the different ways 

 of considering the same subject have created apparent contra- 

 dictions, which have subsequently been reconciled by more accu- 

 rate examination. The first observation of facts is very often far 

 from being accurate ; and, in most cases, this inaccuracy cannot be 

 corrected, till the science itself has attained a higher degree of 

 perfection. The optical and crystallographic inquiries into the na- 

 ture of mineral substances, refer equally to the regular forms 

 which these bodies present. The object of Crystallography is 

 to ascertain this regular form from direct observations ; and that 

 of the department of Optics, which relates to this subject, is to 

 determine the action of regularly crystallised bodies upon light. 

 According to certain general laws, it has been found possible to 

 argue from the optical phenomena to the external forms of mi- 

 nerals, and inversely from these forms to the actions dependent 

 upon them, which affect light in its passage through crystallised 

 substances. 



It may be considered as a general law, as has been shewn by 

 Dr BREWSTER, that the external form of a crystallised body, ex<- 

 hibiting two axes of no polarisation uniformly throughout its 

 whole substance, cannot belong to the tessular, to the rhombo^ 

 hedral, or to the pyramidal system. The observations of crystal- 

 lographers upon a few species, seem still to stand in opposition 



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