Count de Bournon on the Laumojilte. 81 



the short space of time necessary for their examination, to touch 

 them without reducing them to powder. By this mere inattention 

 I have to regret the loss of many very beautiful crystals. As this 

 alteration proceeds, the mineral loses its transparency and at length be- 

 comes of a shining white appearance ; then the faces which belong 

 to the longitudinal planes of the primitive tetrahedral prism, assume 

 a slightly pearly aspect. In proportion to the extent of this altera- 

 tion the degree of cohesion subsisting between the crystalline mo- 

 lecules diminishes. The effect of this first state of alteration is to 

 render the natural joints of the laminse, which are parallel to the 

 longitudinal planes of the prism, much more evident ; and some- 

 times also, but more rarely, those which are parallel to their terminal 

 faces. The alteration increasing, the diminution of cohesion betvv^een 

 the molecules becomes more considerable ; and the separation of the 

 laminse of crystallisation may be easily produced by simple pressure 

 between the fingers. The crystal at length separates spontaneously 

 according to this direction into prismatic fragments, of which many 

 are perfectly regular ; these again subdivide, and ultimately reduce 

 the crystal to a mere powder. This alteration may be prevented, 

 or arrested, by lightly covering the surface of the crystals with gum 

 or with varnish, or by preserving the specimen in distilled water. 



D. SPECIFIC CHARACTERS NOT ESSENTIAL, AND ADMITTING 

 OF VARIATION. 



1. Colour. When it is not altered, this substance has hitherto 

 occurred perfectly colourless. The alteration which it undergoes, 

 renders it opaque, and of a shining white aspect, giving it at the 

 same time a slightly pearly lustre on the longitudinal faces of the 

 prism. 



2, 'Transparency. The laumonite in its perfect and unaltered 



L 



