(54) 
is so great, and their magnitude such, as to allow 
of their being broken into considerable fragments, — 
which exhibit the phcenomena of double refraction 
in much perfection. Some pieces may be obtained 
of the perfect figure of the primitive crystal, which 
will show this quality full as well as the Iceland 
Spar. If we lay one of these over a dot, on a 
piece of white paper, we see the dot doubled. If 
we lay another crystal on the first, with the dia- 
gonals crossing each other, four dots are observed ; 
and, by turning either of the crystals round its axis, 
the dots will appear to change their places, and also 
their numbers ;—sometimes four, sometimes two, and 
at others only one being seen. 
Some are of unusual size, and beautifully trans- 
parent; but these are not common, and are now 
very seldom to be procured. They were generally 
found embedded in clay, detached, rough-coated, 
and not unfrequently of a perfect form. 
Not that fine specimens of any sort are now so 
plenty as they were some time since, and probably 
never will be again; owing to this circumstance,— 
