Dr Brewster m the Structure ApophyllUe* 5 
a greater distance from the summit than formerly, although the 
thickness a b was now 0.09 of an inch, whereas a c was only 
0.02. In some of the pyramidal crystals, I have found the 
force of the axis in the plane of the laminae to be so weak as to 
be nearly ^our or Jive times less than in the quadrangular 
prisms ; and, in several of these last crystals, the maximum tint 
decidedly varies in different parts of the length of the prism, so as 
to produce a succession of coloured hands at the same thickness. 
Of all the crystals which have one axis of double refraction, 
Apophyllite is the only one in which the colour of the rings de- 
viates from those of Newton’s scale. This deviation is very 
common, and, indeed, almost universal, when the rings are 
formed by the joint action of two axes ; and hence it appears 
more than probable, that the single positive, axis of the Fassa 
apophyllite is the resultant of two equal and rectangular nega- 
tive axes lying in the plane of the laminae 
It appears from the preceding observations, that there are at 
least three different kinds of Apophyllite, which may be distin- 
guished from one another by physical characters of a very pal- 
pable kind “I*. The tessellated structure which is possessed by 
one of these kinds is a property so singular and so distinctive, 
that I would propose to mark it by the name of Tesselite. 
The optical structure of apophyllite, which I have now de- 
scribed, may be discovered without the aid of polarised light, in 
specimens of this mineral that are in the course of decomposition. 
In a specimen of the variety called Alhin^ from the cabinet of 
Mr Brooke, I have observed the central rectangle in a state 
of integrity and translucency, while all the surrounding portions 
had assumed a white opacity, from partial disintegration. 
The interior conformation of Apophyllite presents us with the 
new fact in crystallography, that a regular crystalline form arises 
* See PUL Trans^ 1818, p. 247. 
•{■We trust that some able analyst will be induced to examine the chemical 
composition of these different substances. 
Since this paper was printed, I have learned from M. Berzelius, that he has 
analyzed the Apophyllite of Uto ; and, as this distinguished chemist has kindly 
offered to undertake the analysis of the Tesselated Apophyllites from Faroe, we 
expect to be able, in our next number, to publish the results of his analysis* 
