}2 SIR DAVID BREWSTER ON THE EXISTENCE OF CRYSTALS 
possible position in reference to these planes; and we, therefore, cannnot account 
for them by supposing that certain spaces have been left in the crystal, without 
the primitive molecules which ought to have been there deposited. The strata of 
cavities, too, have every possible curvature. From a plane surface they pass into 
a curved one, sometimes of variable curvature, and sometimes of contrary flexure, 
cutting and intersecting each other in the most capricious manner. 
In the shape of the strata the same irregularity presents itself; their outline 
is sometimes rectilineal, sometimes curved, and sometimes singularly irregular, 
In some specimens the whole crystal is intersected with the strata; and it is ex- 
tremely probable, though it is impossible to determine the fact, that in every spe- 
cimen some edge or angle of the stratum touches the surface. 
The succession of the cavities in composing the stratum, and their form in 
relation to the character of the stratum, present interesting phenomena. I have 
found specimens in which the cavities lie in concentric arches, and have their 
sides concentric, and, as it were, a portion of the same arches, as if they had been 
formed under the influence of a rotatory force. In other cases they occupy 
parallel lines, and are sometimes so equidistant that they might be advantageously 
used as micrometers for microscopes. In.one remarkable specimen they radiate 
from a centre, each radiation having a character of its own. One radiation will 
sometimes throw off a diverging branch, while two or more radiations will con- 
verge and then diverge again, subsequently uniting themselves into a single radi- 
ation. 
When different strata of cavities lie parallel to each other in the specimen, 
which they sometimes do, to the number of four or jive, each stratum has generally 
a distinct character; flat and exceedingly thin cavities occupying one stratum, 
very deep cavities occupying another, minute cavities which the highest magni- 
fying powers can scarcely resolve occupying a third, while a fourth consists of 
the most irregular and indescribable forms. 
When the forms of individual cavities are related to that of the stratum 
which contains them, they, of course, cut at all angles the primary and secondary 
planes of crystallization ; and the same is true of insulated cavities of great length, 
which are sometimes turned, and twisted, and bent, in the most capricious 
manner. It is impossible to read these details, and still more so to study the 
phenomena themselves, without being driven to the conclusion, that the strata of 
cavities must have been formed under the influence of forces propagated through 
a soft and plastic mass, and carrying along with them gases and vapours which 
came to a position of rest previous to the regular crystallization of the topaz. 
This conclusion, which I have been led to draw, in another paper, from a series of 
entirely different facts, will be still further confirmed by the phenomena of im- 
bedded crystals, to which I shall have to refer in another section. 
