to his Observations on the Optical System of Mineralogy. 1S9 
glass changes its species with its change of optical character, or 
that optical characters are insufficient to distinguish species? 
What, therefore, does Dr Brewster mean by a Mineral Species ? 
The phrase optical structure , as used by Dr Brewster, does 
not appear to relate properly to structure , but is used to ex- 
press some property of matter which is manifested by it on light , F xc 
and of which it is probable that as little is known as of the na- 
ture of light itself 
The term structure appears, indeed, to be used in several 
senses by Dr Brewster. In the Phil. Trans, for 1818, p. 158. the 
structure of glass is said to be altered by pressure or dilatation. 
It is also stated, that, in glass, the polarising structure 44 de- 
pends entirely on the external form of the plate, and on the 
mode of aggregation of its particles.” The dependence of struc- 
- ture upon external form is not immediately obvious. 
Before we examine the chemical and crystallographical evi- 
dences of the identity of the tesselite and the apophyllite , let us 
«ee what Dr Brewster's own opinions have been concerning the 
tesselite. A paper relating to this mineral was published in 1819? 
in the first volume of this Journal, which Dr Brewster, on an- 
other occasion, says contains an account of his first experiments 
on this remarkable substance. In p. 1. of that volume it is stated, 
that 44 the apophyttites from Faroe crystallise in quandrangular 
prisms , with fat summits , having a truncation upon the angles ; 
and also in single and double four-sided pyramids , the planes of 
which correspond with the small truncations upon the prisms, 
and form angles of (10° * with those on the opposite side.” 
Dr Brewster says in p. 2, 44 Having cut off the slice, about 
jJgth of an inch thick, which formed the summit of a quadran- 
gular prism, I found that it had only one axis of double refrac- 
tion , and produced the same set of coloured rings as apophyllite 
from Fassa ; but upon removing a second slice , of the same size, 
it exhibited the appearance of a tesselated pavement -f-, composed 
of four rectangles , surrounded by a border , and having in the 
centre another rectangle , with its sides opposite the angles of the 
quadrangular prism If the reader will turn to Fig. 1. 
Plate I. of the same volume, he will find the figure which il- 
lustrates this description. In p. 3. it is said, 44 This singular con- 
* This angle is 59 h 16’’ very nearly * 
•f Hence the name ‘ Tesselite/ 
