^ Dr Brewster on the Structure of ApophyUite, 
ated ; but there are others where the surface is less splendent^ 
and where the crystal has the appearance of being more perfect- 
ly formed, as well as more compact and more transparent. 
Having cut off the slice, about the fiftieth of an inch thick^ 
which formed the summit of a quadrangular prism, I found 
that it had only one axis of double refraction, and produced the 
same set of coloured rings, with the same anomalous tints, as the 
apophyllite from Fassa ; but upon removing a second shce of 
the same size, it exhibited the beautiful appearance shewn in 
Fig. 1. This figure, resembling a tessellated pavement, i^ com- 
posed of four rectangles, RT, RV, VS, ST (Plate I. Fig. 1.), 
surrounded by a border MROVNSPT, and having at its centre a 
rectangle ahcd^ with its sides opposite to the angles of the quad- 
rangular prism. The tints depolarised by the four squares^ 
were below the white of the first order, and that of the cen- 
tral rectangle was imperceptible ; but in order to display the 
full beauty of the figure, and to ascertain the character of its 
tints, I placed the slice upon a plate of sulphate of lime, which 
polarised a yellow of the second order, so that the line MN co- 
incided with the principal axis of that crystal. The brilliant 
yellow of the sulphate of lime was raised to a bright red, by 
its union with the tint of the squares RT, VS of the apophyllite, 
and was depressed to a fine blue by the opposite action of the 
tints in the squares RV and ST. The rectangle ahcd re- 
mained yellow, and the border MONP had the same tints as 
the adjacent rectangles. 
Some of the pyramidal crystals are destitute of the tessellated 
structure^ while others exhibit nearly the same phenomena as 
those which we have described ; but owing to the truncation of 
the angles, the pattern has the appearance shewn in Fig. 2, 
where the border shewn in Fig. 1. is completely wanting. By 
taking a succession of slices from the pyramids, we see the struc- 
tures shewn in Figures 3, 4, and 5, and we sometimes obtain the 
appearance represented in Fig. 6, where the central rectangle is' 
wanting, and the tesselce are bounded by irregular curves. 
If we place the slices of apophyllite upon a divided apparatus, 
and expose them to a polarised ray, it mil be found, that at a; 
perpendicular incidence, the first slice, and the rectangle ahcd in 
subsequent slices, have no effect whatever in depolarising the in- 
cident light, but that they acquire it by inclining the polarised 
