Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/93

 Rh 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 becomes 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 alteration the degree of cohesion subsisting between the crystalline molecules diminishes. The effect of this first state of alteration is to render the natural joints of the laminæ, which are parallel to the longitudinal planes of the prism, much more evident; and sometimes also, but more rarely, those which are parallel to their terminal faces. The alteration increasing, the diminution of cohesion between the molecules becomes more considerable; and the separation of the laminæ 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.

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