217 
Dr. Brewster on the laws of polarisation , &c. 
pletely obliterated by the other, and the combined crystals 
would exhibit neither double refraction nor polarisation. 
Hence it follows that zircon , quartz, and ice , superacetate of cop- 
per and lime, and certain specimens of sulphate of potash, form a 
class separate from all the other crystals in our table ; and that 
the polarising force in the one class, is either of an opposite 
nature, or exerted in an opposite direction to the polarising 
force in the other class. This opposition in the action of 
crystals was first observed by M. Biot, in plates cut obliquely 
to the axis ; and by this mode of observation he divided the 
crystals which he examined, into two classes, in the following 
manner : 
REPULSIVE CLASS. 
ATTRACTIVE CLASS. 
Calcareous spar 
Arragonite 
Beryl 
Tourmaline 
Rock crystal 
Sulphate of lime 
Sulphate of barytes 
Topaz 
But since the crystals marked in italics have two or more 
axes of double refraction, and cannot, as we shall afterwards 
show, be ranked in either class, M. Biot’s list, when cor- 
rected, will stand thus : 
REPULSIVE CLASS. ATTRACTIVE CLASS, 
Calcareous spar Rock crystal 
Beryl 
Tourmaline 
agreeing, in so far as it goes, with the more extended table 
which we have already given. Hence it is obvious that M. 
Biot could not have made the experiment which he proposes 
to make, at the end of the passage just quoted; namely, to 
MDCCCXVIIL F f 
