200 PHYSIOGRAPHY. CLASS 11. 



OBSERVATIONS. 



1. The substances hitherto comprised under the name of 

 Mica, do not all belong to the [species of rhombohedral 

 Talc-mica ; but it would be impossible, from the present im- 

 perfect state of our information, to draw clear lines of se- 

 paration between the various species designated by that 

 name. Although in several of them the system of crystal- 

 lisation be known, yet we are not acquainted with the re- 

 spective series of crystallisation, that is to say, with the 

 angles of the fundamental rhombohedrons or scalene four- 

 sided pyramids ; and the distinctive characters which then 

 remain are not in general sufficient to give security and 

 evidence to the determination of the species. The optical 

 researches, however, of Dr BREWSTER and M. BioT,have 

 shewn that the various kinds of Mica differ considerably in 

 their action upon light, some of them possessing only one 

 axis of double refraction, and shewing one system of co- 

 loured rings, while others possess two axes, and shew there- 

 fore two systems of coloured rings. Among the first again, 

 some possess a positive optical axis like rhombohedral 

 Quartz, though the greater part of them exhibit a negative 

 one, like rhombohedral Lime-haloide. The resultant axes, 

 or those of no polarisation in the other, are inclined to 

 each other at various angles, and, besides, are situated in 

 planes perpendicular to the laminae, which in some of them 

 pass through the long diagonal, in others through the 

 short diagonal of a rhombic prism of 120 and 60, supposed 

 to result from the enlargement of four of the lateral faces 

 of the six-sided laminae. Count BOURNON assumes an 

 oblique rhombic prism for the primitive form of Mica. 

 According to Mr SORET, who likewise supposes the primi- 

 tive form of certain varieties of mica to be an oblique 

 prism, the plane of the resultant axes passes in these 

 through the short diagonal of the base, and this takes place 

 in several micas from Sweden and Siberia. In others, 

 where the primitive form is a right prism, the plane of the 

 resultant axes passes through the long diagonal of the base, 



