to fiis Observations on the Optical System of' Mineralogy. 191 
of the foundation-stone y the roof and the central pillar ? — and 
at what angle do its planes incline to each other ? What are the 
plane angles of the section , or imaginary base , of the four tra- 
pezoidal solids f Are these angles constant in the same crystals , 
and in different crystcds ; and are they constant throughout the 
zvhole length of these imaginary forms ? What is the figure , 
and what are the angles , of the primitive form of the particles 
of which the trapezoidal solids are composed ? Are the trian- 
gular prisms composed of particles similar to these f— and horn 
are they arranged in the construction of each of these two sets 
of prisms ? When information is afforded to the crystallographer 
on these points, he will be enabled to ascertain whether the crys- 
tals fall within the range of ordinary laws of structure or not. 
But we may regard the necessity of these inquiries to be near- 
ly superseded by Dr Brewsters latest opinion relative to this 
mineral, as it appears in p. 364. of No. 18. of this Journal. In 
looking through the different papers in which Dr Brewster has 
alluded to the tess elite ^ it is not easy to collect whether he attri- 
butes its extraordinary optical character to its structure , or to- 
the nature of its elements. The quotations I have just given 
would seem to refer the optical phenomena principally to its 
structure , but in the following passage they are more particularly 
attributed to the properties of its elements. In this passage, it 
is stated, that u tesselite is a substance built up , as it were , of 
the most singular elementary parts , all of which parts have 
different optical and mechanical properties.'''’ A climax, indeed, 
in the description of this mineral, which transcends all Dr Brew- 
ster’s previous views regarding it. 
u Hence, 1 ” he says, “ an analysis of tesselite resembles the 
analysis of a bird, the feathers, flesh and bones of which are all 
pounded together in a mortar.” But if this last description of 
the substance were correct, and the mortar were to contain the 
whole of the animal, vegetable, and mineral kingdoms together, 
the mass would be homogeneous in comparison with tesselite . 
Let us, however, quit these regions of imagination, and exa- 
mine the evidences of the chemical distinction of tesselite from 
apophyllite . 
In page 364. of No. XVIII. of this Journal, Dr Brewster 
quotes a paragraph of mine, stating, that “ Tesselite does not. 
