a stone so named by Mr Werner, inspector of the mines of Freiburg, brought by Colonel Prehn from the Cape of Good Hope. In the first volume of Chemical Annals there is a chemical analysis of this stone extracted from the Acta Naturae Curiosorum, Berlin, tom. viii. p. 211. an. 1788, part 2. by Klapproth; from which it appears, that 100 parts of prehnite contain silice 43½ grains, alumine 30½, lime 18½, oxyd of iron 5½, water and air 1½.

Total 100

Colonel Prehn gave it the name of emerald, and Mr Bruckmann adopted that denomination, but changed his opinion on considering that it had neither the hardness, the bright green colour, nor the property of crystallizing in hexagonal prisms like the emerald. The Dutch dealers call it chrysoprasus of the Cape; but chrysoprasus is nothing but quartz tinged green by the oxyd of nickel. Professor Haquet, in the 4th volume of the Berlin Transactions, has named it crystallized prasus. In the 8th volume, however, Mr Bruckmann considers it as a crystallized felt-spar. Mr Sage calls it chrysolite. Mr Rone de l'Isle classes it among the schorls. To this classification Mr Klapproth objects, and is rather disposed with Mr Werner to consider it as a zeolite; on the whole, he thinks it may be conveniently ranked between zeolite and schorl. Mr Haffenratz published in the Journal de Physique for February 1788 an analysis of the same stone, under the title of Pierre Silice, calcaire, alumineuse, &c. &c. de couleur verte, &c. And according to his results, which are somewhat different from those of Mr Klapproth, and obtained by a different process, it contains, silice 50, lime 23.4, alumine 20.4, oxyd of iron 4.9, water c.9, magnesia o.5 = 100. The specific gravity of this stone, according to the experiments of Mr Briffon, is 2.9423. The direction of its crystals, made by Abbé Haüy and Mr Haffenratz, discovered one angle of 60 degrees, as in schoerls; but it has hitherto been impossible to determine the others. The Abbé Haiyi has convinced himself, that it bears no resemblance to zeolite in its crystals. This stone scratches glass, and is itself scratched by rock crystal.