38 
NORTH GALLERY. 
[upper 
especial!}^ from its power of absorbing one of the two " polarised " rays 
into which every single ray of light is divided that penetrates it in any 
but the loiig direction of its crystal. The colour suite of the Tourma- 
line is extensive and well illustrated, but the magnificent specimen of 
the Rubellite variety (from Ava) is unique in size and perfection. 
The next Table contains the compounds of Titanic acid, and after 
these commences the long series of The Carbonates which occupy the 
Cases as far as 51. The isomorphous character of the Oxides, Lime, 
.Baryta, Strontia, oxide of Lead, Magnesia, and the protoxides of Iron 
and of Manganese is illustrated by species in which their carbonates 
crystallize in equivalent forms, or are united in the same mineral, 
while also the trimorphous nature of these carbonates finds examples 
in the three series of forms in which the Calcite group, the Aragonite 
group, and the mineral Barytocalcite are severally seen crystallized. 
The first set of forms is exhibited in a fine collection of crystals of 
Iceland Sijar or Calcite, the RJiombohedral form of the Carbonate of 
Lime, the conspicuous double refraction of which mineral is exhibited 
by a large specimen of it, in illustration of a property common to all 
crystals except those belonging to the crystalline system which repre- 
sents the ccmpletest symmetry (that, viz., to which the cube belongs). 
It is this Carbonate of Lime that in its impure forms constitutes the 
vast series of Limestone rocks, while its kindred mineral, Spathose 
Iron (the Carbonate of that metal) or Clialyhite, forms a large per- 
centage of the " Clay Iron-stone " from which the furnaces of Wales, 
Scotland, &c., are mainly fed. Among the Carbonates combined with 
water and otlaer substances may be noticed the Copper-ores, Malachite 
and Chessylite, as also the scarce mineral Parisite, containing some of 
the rarest of the earths, the metals of which are partly in combination 
as fluorides. The dimorphous sulphato-tricarbonate of Lead in its 
two forms of Leadhillite and Suzannite links the Carbonates with the 
next group. 
This group, which occupies Cases 52 to 55, consists of the Salts, 
of Sulphuric and of other Acids allied to it by chemical type and by 
the crystallographic relationship of their compounds. The rare series 
of the Chromates, Molybdates and Tungstates find their place here 
with minerals of the greatest scarcity and interest, the Vanadates,. 
Columbates, Tantalates, &c. The Sulphates are represented by 
Barytes, Celestine, Anhydrite, kc. It is the mineral Gypsum, or Se- 
lenite, which, by the loss of its water when heated, becomes " Plaster 
of Paris."' 
The Phosphates and Aesentates, an extensive class, containing 
among its species many rare minerals, present themselves in their 
numerous forms and beautiful colours in Cases 56 and 57. The iso- 
morphism of Phosphoric, Arsenic and Antimonic acids (the last occurs 
in liomeite), their frequent association with Chlorides and Fluorides^ 
and the hirge and complicated range of chemical type which these 
tribasic acids exhibit, are the remarkable features of the class of 
Minerals displayed in these Cases. 
They are succeeded at present by the Fluorides, conspicuous among 
