ulphur dioxide yields the "bisulphite of lime" of commerce,
which is used in the "chemical" manufacture of wood-pulp for paper making.

_Calcium sulphate_, CaSO_4, constitutes the minerals anhydrite (_q.v._),
and, in the hydrated form, selenite, gypsum (_q.v._), alabaster (_q.v._),
and also the adhesive plaster of Paris (see CEMENT). It occurs dissolved in
most natural waters, which it renders "permanently hard." It is obtained as
a white crystalline precipitate, sparingly soluble in water (100 parts of
water dissolve 24 of the salt at 15 deg.C.), by mixing solutions of a sulphate
and a calcium salt; it is more soluble in solutions of common salt and
hydrochloric acid, and especially of sodium thiosulphate.

_Calcium silicates_ are exceptionally abundant in the mineral kingdom.
Calcium metasilicate, CaSiO_3, occurs in nature as monoclinic crystals
known as tabular spar or wollastonite; it may be prepared artificially from
solutions of calcium chloride and sodium silicate. H. Le Chatelier
(_Annales des mines_, 1887, p. 345) has obtained artificially the
compounds: CaSiO_3, Ca_2SiO_4, Ca_3Si_2O_7, and Ca_3SiO_5. (See also G.
Oddo, _Chemisches Centralblatt_, 1896, 228.) Acid calcium silicates are
represented in the mineral kingdom by gyrolite, H_2Ca_2(SiO_3)_3.H_2O, a
lime zeolite, sometimes regarded as an altered form of apophyllite
(_q.v._), which is itself an acid calcium silicate containing an alkaline
fluoride, by okenite, H_2Ca(SiO_3)_2.H_2O, and by xonalite 4CaSiO_3.H_2O.
Calcium silicate is also present in the minerals: olivine, pyroxenes,
amphiboles, epidote, felspars, zeolites, scapolites (_qq.v._).

_Detection and Estimation._--Most calcium compounds, especially when
moistened with hydrochloric acid, impart an orange-red colour to a Bunsen
flame, which when viewed through green glass appears to be finch-green;
this distinguishes it in the presence of strontium, whose crimson
coloration is apt to mask the orange-red calcium flame (when viewed through
green glass the strontium flame appears to be a very faint yellow). In the
spectroscope calcium exhibits two intense lines--an orange line ([alpha]),
([lambda] 6163), a green line ([beta]), ([lambda] 4229), and a fainter
indigo line. Calcium is not precipitated by sulphuretted hydrogen, but
falls as the carbonate when an alkaline carbonate is added to a solution.
Sulphuric acid gives a white precipitate of calcium sulphate with strong
solutions; ammonium oxalate give