1824.] Scientific Notices — Mineralogy. 393 



thrown out on the 24th of October ; different varieties of quartz 

 (flint andmenilite, and specimens passing from those substances 

 to a lava of amphigene and pyroxene) ; phosphate of lime in 

 hexahedral prisms ; melilite in cubes, similar to those from 

 Capo di Bove (the last two found in a current (of lava?) on the 

 declivities of Mont Somma, above Pollena) ; gehlenite, similar 

 to that from Fassa ; specular iron, octohedral oxidulated iron ; 

 antimonial iron and glass of antimony combined with a little 

 osmium. — (Bulletin des Sciences Naturelles.) 



14. On the Contractions of Crystals bij Heat. 



M. Mitscherlich has observed that the mutual inclination of 

 the faces of Iceland spar vary in a sensible manner by the effect 

 of heat, and that between 0° and 100° (32° and 212° Fahr.) the 

 change from the dihedral angles to the extremities of the axis of 

 the rhomboid is 8^-'. It results from this, that if we suppose the 

 dilatation of the crystal perpendicular to its axis to be nothing, 

 its cubic dilatation should still exceed that of glass by nearly 

 one-half; but on measuring the cubic dilatation of Iceland spar 

 with M. Dulong, M. Mitscherlich found, on the contrary, that 

 it is less ; which leads to the singular consequence, that while 

 heat dilates the crystal in a direction parallel to its axis, it must 

 cause it to contract perpendicularly. M. Mitscherlich has 

 ascertained this to be the fact, by measuring with a spherometer, 

 at different temperatures, the thickness of a plate of Iceland 

 spar, cut in a direction parallel to its axis. It is very probable 

 that sulphate of lime may present an analogous phenomenon, 

 but the reverse of the preceding ; that is, that elevation of tem- 

 perature may produce a sensible contraction in the direction of 

 its axis. A. F. — (Annates de Chimie.) 



15. On the Inclination of the Line dividing the Optical Axes of 



certain Crystals. 



It is known that the optical axes of crystals, improperly called 

 crystals wilh two axes, do not coincide with the axes of crystal- 

 lization ; and it has been hitherto regarded as a general law,, 

 that the right lines which divide the an°;le contained between 

 the optical axes into two equal parts must be equally inclined on 

 the corresponding faces of the crystal. M. Mitscherlich has 

 found that these lines, symmetrical with respect to the double 

 refraction, are not always so with respect to the faces of the 

 crystal, and that in some salts, such as the sulphate of magnesia, 

 they are more inclined to one side than the other, without any 

 want of symmetry in the crystalline form, leading one to presup- 

 pose any such deviation. A. F. — (Annales de Chimie.) 



