]2 SJR DAVID BREWSTER ON THE EXISTENCE OF CRYSTALS 



possible posit ion in reference to these planes; and we, therefore, cannnot account 

 for them b^^ 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 vai'iable curvature, and sometimes of contrary flexure, 

 cutting and intersecting each other in the most capricious manner. 



In the shape of the strata the same h-regularity presents itself; their outhne 

 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 eveiy spe- 

 cimen some edge or angle of the stratum touches the sm-face. 



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 

 fi'om 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 conr 

 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 ovftfe, 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 aU angles the primary and secondary 

 planes of crystaUization ; 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, Avithout being driven to the conclusion, that the strata of 

 cavities miist have been formed uTuler 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 difterent facts, will be still further confirmed by the phenomena of im- 

 bedded crystals, to which I shall have to refer in another section. 



