oxide. It is also used for glazing pottery, in glass-making and the
glazing of linen.

Boric acid (q.v.) being only a weak acid, its salts readily undergo
hydrolytic dissociation in aqueous solution, and this property can be
readily shown with a concentrated aqueous solution of borax, for by
adding litmus and then just sufficient acetic acid to turn the litmus
red, the addition of a large volume of water to the solution changes
the colour back to blue again. The boric acid being scarcely ionized
gives only a very small quantity of hydrogen ions, whilst the base
(sodium hydroxide) produced by the hydrolysis occasioned by the
dilution of the solution, being a "strong base," is highly ionized and
gives a comparatively large amount of hydroxyl ions. In the solution,
therefore, there is now an excess of hydroxyl ions; consequently it
has an alkaline reaction and the litmus turns blue.

_Mineralogy._--The Tibetan mineral deposits have been known since very
early times, and formerly the crude material was exported to Europe,
under the name of _tincal_, for the preparation of pure borax and other
boron salts. The most westerly of the Tibetan deposits are in the
lake-plain of Pugha on the Rulangchu, a tributary of the Indus, at an
elevation of 15,000 ft.: here the impure borax (_sohaga_) occurs over an
area of about 2 sq. m., and is covered by a saline efflorescence;
successive crops are obtained by the action of rain and snow and
subsequent evaporation. Deposits of purer material (_chu tsale_ or water
borax) occur at the lakes of Rudok, situated to the east of the Pugha
district; also still farther to the east at the great lakes Tengri Nor,
north of Lhasa, and several other places. More recently, the extensive
deposits of borates (chiefly, however, of calcium; see COLEMANITE) in
the Mohave desert on the borders of California and Nevada, and in the
Atacama desert in South America, have been the chief commercial sources
of boron compounds. The boron contained in solution in the salt lakes
has very probably been supplied by hot springs and solfataras of
volcanic origin, such as those which at the present day charge the
waters of the lagoons in Tuscany with boric acid. The deposits formed by
evaporation from these lakes and marshes or salines, are mixtures of
borates, various alkaline salts (sodium carbonate, sulphate, chloride),
gypsum, &c. In the mud of the lakes and in the surrounding marshy soil
fine