Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/94

 82 state is beautifully transparent, but it is extremely rare to meet with it in this condition, in consequence of the great facility and rapidity with which its alteration proceeds.

3. Phosphorescence. This substance does not possess any sensible phosphorescence; however, as it is accompanied, and even intermixed, with a highly phosphorescent lamellar carbonate of lime, the luminous appearance of which is, a bright reddish yellow or orange, it is liable to have some portions of this carbonate of lime interposed throughout its substance, which is the cause of its being sometimes observed, that fragments of it when submitted to trial give traces of a slight phosphorescence.

If this substance offer a striking peculiarity, and one very proper to entitle it to a distinct place in the classification of mineral substances, by the facility with which simple exposure to the atmosphere causes it to undergo disintegration, and finally to fall into powder; its crystallisation, which cannot be referred to that of any other known mineral, adds still more to the singular characters which the laumonite exhibits. This crystallisation generally appears in the state of crystalline masses, often of considerable size, and which at first sight present only a deeply striated or fasciculated surface; but these same masses, which apparently have no determinate crystalline forms, on being broken become immediately a rich field of observation by the great quantity of extremely perfect and diversified crystals, which the portions into which they divide, afford. They are therefore, at least with regard to a great number, merely a confused aggregation (arranged however in the direction of their prisms) of crystals perfectly formed, and of different shapes, piled one above