148 



ble into laminae, much resembling carbonate of lime,, 

 and gives a double refraction through the rectangular 

 faces only, but somewhat weaker than that of carbo- 

 nate of lime. This is a curious circumstance, and perhaps 

 has not yet been noticed. It may lead to the true nature 

 of double refraction. Haiiy had recourse to the ingenious 

 method of forming artificial faces to discover this pro- 

 perty. 



Our specimen is on an iron ore from Lancashire, and is 

 as near the primitive as is generally seen in Great Britain, 



TAB. LXXI. 



The upper fragment is in the form of the nucleus, or 

 an upright parallelopiped ; and as the faces are the same with 

 the primitive, it is placed with the upright faces on a line, 

 to show the refraction is not double in that direction : and 

 it is to be observed that it requires a large depth of crystal 

 to see the refraction through the other faces without the 

 assistance of a lens. The flaws show the prismatic tints, 

 like other laminated crystals ; this shows the depth of the 

 flaw, and it is so sufficiently elastic that we can, by pres- 



