Cryjlals of Oxidated Tin Ore, 155 



of each refpedlively, produce a great variety of forms, while 

 other varieties arife from difference in colour and tranfpa- 

 rency to which all thefe cryftallifations are fubjeft. 



Tbe macles of De I'lfle, which Hauy has called hemi- 

 irope, or half-turned, may be added as a fupplementary 

 clafs. Of the term made, De I'lile gives this explanation: 

 ** When in any cryftal whatever one or more re-entering 

 or internal angles are obferved, we may conclude that it is 

 net a fimple cryftal, but a group of two or more ; or even 

 two turned or reverfed moieties of the fame cryftal." Dc 

 rifle was indebted to a Mr. Lermina for determining the 

 direftion of the fetlion which produced the hemitrope cryf- 

 tals of tin, which he has fliewn to be parallel to two oppoiite 

 edges of the o6taedron. Imagine the primitive odlaedron to 

 be divided into halves by a plane pafllng diagonally through 

 it, from the centre B, fig. 24, of one of the edges of the fu- 

 perior pyramid, to the centre B of that edge of the inferior 

 pyramid which is diagonally oppofite ; the plane wiil pafs 

 through two oppoiite angles FE of the bafe, dividing on each 

 pyramid two adjoining faces into four fcalene triangles, and 

 leaving two adjoining faces perfeft, but reverfed on one py- 

 ramid in refpeft to the other. The plane FBEB' of thia 

 fcftion is a rhomb of 60' and 126^. 



The prifmatic cryftals appear to be as fufceptible of that 

 fpecies of accumulation which produces the hemitrope as the 

 oftacdron, and thefeftion is always parallel to the one deter- 

 mined above. If the prifm is fhort, and terminated by two 

 pyramids, the fcition will pafs through the prifm into both 

 pyramids, and be irregularly oftagon ', but if the prifm is 

 long enough to permit the feftion to pafs through it without 

 encroaching on the pyramids, it will be a rhomb of 70' and 

 110°; the fame as the fedion of the primitive o6laedron, per- 

 pendicular from the apex, and diagonally acrofs the bafe. 



I have fomc cryftals which are produced by a double fee- 

 lion, and both ends turned on the middle part; others which 

 appear to be only portions taken off the angles at the junc- 

 tional 



