90 



MINERALOGY. 



the Ortboclase called " Valencianite," from Mexico; also, fine speci- 

 mens of Amazonite and other varieties of microciine felspar. 

 Cases Dichroite (the Sapphire d'Eau of jewellers) is remarkable for its 



29 to 82. pleiochroism, a character due to the different degrees in which the 

 crystal absorbs the light of different colours according to the planes 

 of their vibration ; the crystal when looked through perpendicularly 

 to the basal face 001 is of a rich blue, perpendicularly to the faces 

 010 and 100 it is of a bluish white, and of a pale straw colour respec- 

 tively. The Beryl includes the Emerald, and also the Aquamarine of 

 the jewellers ; it is an alumino-glucinum silicate, the Aluminium being 

 in the Emerald apparently displaced to a minute amount by Chromium. 

 Euclase is a mineral composed of the same elements, and containing 

 a small quantity of water: the specimens of it from Siberia are of high 

 interest. These are followed by hydrated silicates, including a very 

 complete collection of the Zeolites, among which the Mesolite from 

 India, the Scolecite from Iceland and India, and the Edingtonite 

 from Scotland are remarkable. Here are exhibited remarkably fine 

 specimens of Harmotome, of Stilbite, of Waluewite and Clinochlore, 

 oi' Cronstedtite and Pyrosmalite : large specimens of the latter mineral 

 are shown in a glazed end of Case 33. 

 Oases The silicates proper are succeeded by minerals in which 



3 K 6 ' silicates are associated with boric-oxide or borates. Among these 

 the Tourmalines present a rich assortment of valuable and beautiful 

 specimens, conspicuous for crystals of Rubellite from Siberia and 

 Ava. Two very fine specimens of the Rubellite from the latter 

 country are seen in this Case. The one remarkable for its magnitude 

 and form was brought from Ava by Colonel Symes, to whom it 

 was a present from the King of Ava. The other, also a very large 

 specimen, and of deep colour, was presented in 1809 by C. S. J. L. 

 Guthrie, Esq. These are succeeded, in Case 34, by a class of minerals 

 of great mineralogical interest, containing some of the rarest of the 

 elements, and themselves of rarity; much uncertainty, however, still 

 attaches to the chemical formulae of several of these species. The 

 titanates, the tantalates, and niobates, and these combined with silicates, 

 zirconates, and stannates, thus link the silicates to the molybdates 

 and tungstates, and these, in turn, are followed by the class of 

 ohromates and the sulphates. The suite of specimens of Perofskite, 

 the crystals of Eudialyte, of Columbite, of Fergusonite, of Pyrrhite, 

 and of iEschynite, and the specimens of Tscheffkinite, are especially 

 observable for their excellence or their rarity. 

 Cases Among the anhydrous species in the sulphates, attention may be Sulp 



35 to 40. called to the specimens of Celestine (strontium sulphate) from near 

 Bristol and Sicily, and to the Anglesite (lead sulphate) from Derby- 

 shire, as also from Pennsylvania and Monte Poni. Gypsum, or 

 Selenite, the hydrated calcium sulphate, is an important mineral as 

 yielding Plaster of Paris by the expulsion of its water. A magnifi- 

 cent specimen of this mineral, as remarkable for its size as for the 

 grouping of its crystals, presented by His Royal Highness the late 

 Prince Consort, ornaments a window in the Pavilion. It was found 

 at Reinhard's-bruhn, Saxe-Coburg. 



