gg ON SPURIOUS CRYSTALS. 



found in shown US in the crystals of the gcodes of Oberstelii this very 



^"^"^ ■ secondary form, the structure of which, as ascertained by Mr. 



Haiiy, is derived from the primitive rhomboid of quartz. But 

 since this variety of form, which has not escaped the atten- 

 tive eye of Mr. Tondi, occupies a place in the series of forms 

 of quartz, the difficulty vanishes, analogy resumes all its 

 weight, and the origin I ascribe to the regular forms of steatite 

 retains it probability. 

 Argument ^ '^^^ ^^^^^ ^^ crystallization have been appealed to in favour 

 from the laws of the opinion I combat. On breaking the steatite of Bay- 

 tion. reuth, we discover in it parts, which have the form of the 



rhomboidal calcareous spar. It is in feet the primitive rhom- 

 boid of carbonate of lime, which has been mentioned above 

 as 6ne of the forms, under which steatite sometimes presents 

 itself. " Now it has been said, rhomboidal molecules are capa- 

 ble of producing the prismatic form of rock crystal, and that 

 of the inverse calcareous spaf, the muriatic calcareous spar 

 of del'Isle: therefore; the forms observed in steatite may be 

 its own. It is very true, that the obtuse rhomboid of I01|°, 

 similar to that of carbonate of lime, performing the office of 

 a nucleus and substractive molecule, may produce the hex- 

 , aedral prism of rock crystal. It does this in the prismatic 



carbonate of lime, by means of a decrement on the infei'ior 

 angle of the nucleus in which two rows of molecules are sub- 

 jproof against tracted ; and this law is general for every rhomboid. But it 

 ^ ' cannot produce the hexagonal pyramid, which terminates the 



prismatic hyaline quartz, with the same incidences which are 

 constantly found in the quartz ; as these require for the pri- 

 mitive form and subtractive molecule a slightly obtuse rhom- 

 boid only, the a>igle of which is about 94°. 

 Carbonate of Mr. Hericart Thury, engineer of mines, has found near 



lime crystal- Qjenoblc indeed carbonate of lime crystallized in a hexaedral 



lized m the •' 



same figure, prism with a pyramidal summit of six triangular faces ; but 



but with dif- jj^ig £q^,j^ j^^g nothing in common with the prismatic quartz, 

 the crystals being altogether different, both in respect to the 

 incidences of the faces, and the values of their angles. It 

 differs from the prismatic hyaline quartz, as the greenish yel- 

 low phusphate of lime in hexaedral prisms terminated by 

 hexagonal pyramids, the spargektcin of Werner, differs from 



tbe 



