MINEKALOGY. 



77 



crystallographic forms which it takes the skill of a crystallo- 

 grapher to reduce to a common symmetry ; or, again, as in 

 one locality in Iceland, it occurs in large masses of limpid 

 crystal, conspicuous for its double refraction, and for the 

 invaluable means which, in the hands of Bartholinus, Huyghens 

 and Fresnel, it has thereby afforded for the investigation of 

 the properties of light ; or, again, in its softer form of chalk, 

 it subserves many a domestic use. 



Here also are to be found rough and cut specimens of the 

 precious stones, among which may be mentioned the Diamond, 

 a crystallised form of the element Carbon ; the Balas ruby 

 and the Spinel ruby, a compound of alumina and magnesia ; 

 the Chrysoberyl and Alexandrite, a combination of alumina 

 and giucina ; the Sapphire and Ruby, the sesquioxide of 

 aluminium ; the Hyacinth and Jargoon, a compound of silica 

 and zirconia ; the Amethyst, Sard, Plasma, Prase, Chalce- 

 dony, and Noble Opal, which are varieties of silica or quartz ; 

 the Chrysolite and Peridot, a silicate of magnesia and iron ; 

 the Garnet with a varied composition as above mentioned ; 

 the Beryl, Emerald and Euclase, compound silicates of 

 aluminium and glucinum ; the Tourmaline and Rubellite, a 

 borosilicate of several bases ; the Lapis-Lazuli, a complex 

 combination of silicate and sulphate ; and the Turquoise, a 

 hydrated phosphate of aluminium. 



Nor from the list of the interesting contributions of a 

 mineral collection should be omitted the series of meteoric 

 bodies which have come to this earth from the regions of 

 space. These strange masses of metallic iron, more or less rich 

 in nickel, or of stone impregnated with the same metallic 

 material, serve as witnesses that the same laws of chemical 

 combination and of crystallographic symmetry, and the 

 same elements, of which our own world is built up, per- 

 vade the regions of space through which these masses of 

 matter have wandered swiftly till, entangled in our atmosphere, 

 their course has been arrested and they have fallen to the 

 earth with startling accompaniments of explosion, fusion, 

 and dissipation of their material, as a consequence of the 

 enormous temperature for which they have exchanged an 

 often more than planetary velocity. 



g 2 



