to his Observations on the Optical System of ‘ Mineralogy. 193 
Silex. 
Lime , 
Potash . < 
FL Ac. 
Water. 
Tesselite, Faroe, Berzelius, 52.38 
24.98 
5.27 
0,64 
16.20 
Apophyllite, Uto, Berzelius, 52.19 
24.71 
5.27 
0.82 
16.20 
Tyrol, Gmelin, 52.38 
24.86 
5.27 
— 
16,19 
From this statement, the reader will judge how far I am jus- 
tified in concluding, that, “ chemically , the Tesselite does not ap- 
pear a distinct species and to what confidence Dr Brewster's 
positive denial of the accuracy of this conclusion is entitled. 
I have re-examined, with some attention, the crystallogra- 
phical characters of the apophyllites, and I must confess myself 
unable to discover in what essential particulars the tesselite dif- 
fers from the others. If astonishment at the splendour of the 
optical imagery of tesselite had not interfered with Dr Brewster's 
observation of other matters, he might have perceived, that the 
transverse cleavages were all perpendicular to the axis of the 
prism , ami were consequently parallel to each other . If the 
crystals had been composed of pyramidal primitive forms in 
their interior, and prismatic primitive forms surrounding these, 
it is not probable that they would all have lain in precisely the 
same planes. But if Dr Brewster had looked a little closer to 
the laminae obtained from cleavage, he might possibly have per- 
ceived that his wall and his nine prisms, were all traversed by in- 
dications of continuous natural joints , parallel to the edges and 
diagonals of the plate ; for such joints are very apparent in se- 
veral of the slices I have examined, and which contained outlines 
analogous to those of Dr Brewster's trapezoidal solids. These 
outlines do not, however, appear to be quite as regular as Dr 
Brewster has drawn them ; nor do they seem to be always re- 
ferable to precisely the same cause; for, upon breaking several 
crystals in which they were visible, I have observed that they 
were the edges of planes of composition, which were sometimes 
parallel to the lateral planes of the prism, and were sometimes 
oblique, — those parallel to the lateral planes having their surfaces 
striated longitudinally, and the oblique planes being obliquely 
striated. It seems probable, that the outlines supposed to be- 
long to the four rectangular solids, might result from the transi- 
tion of a pyramidal to a prismatic crystal * ; or rather, perhaps, 
from the joint action of the two forces or circumstances, be they 
* This is a fact of frequent occurrence among crystals, particularly in carbonate 
of lime, where regularly-formed figures are enveloped by new deposites upon their 
VOL. XI. NO. 2L JULY 1. 1824. N 
