1859.] cu investigated by Liyht. 97 



of every axis of elasticity in it — were shown to be represented by a spheie. No 

 more was said of this system. 



The pyramidal and rhombohedral systems were morphologically associated, by 

 the fact of each having one morphological axis, round which a perfect symmetry 

 reigned— though crystallographic laws ranged the facettes in multiples of 4 

 round the axis of the first, and of 3 round that of the second. The elasticity in these 

 crystals was represented by a spheroid —prolate ''called negative), or oblate (called 

 positive ), accoi-ding as the elasticity was greater in an axial or an equatorial direction. 



The prismatic system was represented, asregaided its elasticity, by an ellipsoid, 

 a figure whose axes are all unequal, though rectangular. 



The oblique systems, also approximately represented by ellipsoids, were 

 discussed later. 



These preliminary statements having been gone through, the speaker 

 entered on the question of the internal molecular arrangement of the 

 crystal, and first briefly reviewed what has been established regarding 

 the physical relations of crystals. 



lie introduced the name of Professor Grailich, of Vienna, in con- 

 nection with this subject as one who, with his pupils, had worked over 

 the whole of this large field of research, and had both added much to the 

 facts themselves, and contributed greatly towards the extension of the 

 theoretical views that must be called in to explain them. Some of the 

 more recent of these results of Professor Grailich and his collaborateur. 

 Dr. Viktor von Lang, formed the chief subject matter of what followed. 



The experiments made hitherto to determine the action of a crystal 

 upon different physical j)owers, were then reviewed and shown to lead 

 to the general conclusion that there is a very close relation between its 

 morphological (and therefore geometrical) symmetry and its physical 

 properties. 



The mechanical elasticity possessed by the crystal in its different 

 directions, may be examined through its cohesion, as indicated by its 

 cleavage, its hardness, its acoustic properties, &c. 



The Cleavage, where it exists in a crystal, invariably occurs in 

 the direction of actual or of (in obedience to the great crystallographic 

 law), possible crystal-planes, and is furthermore found to follow planes 

 represented by very simple indices. Besides this relation between the 

 direction in which the crystal splits and its crystallographic form, there 

 is also exhibited a remarkable relation between the degrees of facility 

 in which the substance yields to the cleaving force and the symmetry 

 of the crystal. Such crystals as have equal parameters exhibiting equal 

 facility of cleavage in three directions ; such as have two only exhibit 

 equally easy cleavage in two directions. 



The Hardness of a crystal also varies on its different facettes, and 

 in different directions on the same facette ; and it would seem to be a 

 general law that the greatest hardness is exhibited in a direction and 

 on a plane at right angles to that of cleavage, and that the hardness 

 graduates in its degree in proportion as the plane experimented on 

 tends to coincide with that plane. [Frankenheim, Franz, Grailich 

 and Pekarck.] 



The Acoustic characters of a crystal are of an analogous kind. 

 So far as they have been investigated they seem to follow the law of its 



Vol. II. (No. 29.) h 



