density of 10° to 11° Baumé. It is allowed to settle in the vessel C, in which it cools and crystallises, yielding (not quite pure) crystalline boric acid. At temperatures above 100°, for instance, with superheated steam, boric acid volatilises with steam very easily. [Illustration: FIG. 81.--Extraction of boric acid in Tuscany.] If boric acid be introduced into an excess of a strong hot solution of sodium hydroxide, then, on slowly cooling, the salt NaBO_{2},4H_{2}O crystallises out. This salt contains an equivalent of Na_{2}O to one equivalent B_{2}O_{3}. It might be termed a neutral salt did it not possess strongly alkaline reactions and easily split up into the alkali and the more stable borax or biborate of sodium mentioned above, which contains 2B_{2}O_{3} to Na_{2}O.[5] This salt is prepared by the action of boric acid on a solution of sodium carbonate. Borax may be perfectly purified by crystallisation. If a saturated and hot solution of borax be mixed with strong hydrochloric acid, common salt and a normal crystalline hydrate of _boric acid_ are formed. The composition of this hydrate is B(HO)_{3}, according to the form BX_{3}--that is, of the composition B_{2}O_{3},3H_{2}O. This is the easiest method of obtaining pure boric acid. The water is easily expelled from this hydrate; it loses half at 100° and the remainder on further heating, and the remaining B_{2}O_{3} or boric anhydride fuses at 580° (according to Carnelley), forming at first a ductile (easily drawn out into threads), tenacious mass and then a colourless liquid solidifying to a transparent glass, which absorbs moisture from the atmosphere and then becomes cloudy.[6] Only the alkaline salts of boric acid are soluble in water, but all borates are soluble in acids, owing to their easy decomposability and the solubility of boric acid itself. Although boric anhydride, B_{2}O_{3}, absorbs 3H_{2}O from damp air, still in the presence of water it always[7] combines with a less quantity of bases (borax only contains 1/6). However, fused boric anhydride forms a crystalline compound with magnesium of the same type as the hydrate (MgO)_{3}B_{2}O_{3} (Ebelmann), and even with sodium it forms (Na_{2}O)_{3}B_{2}O_{3} or Na_{3}BO_{3} (Benedict). As a rule, the salts of boric acid contain less base, although they are all able to form saline compounds with bases when fused. Generally, vitreous fluxes are formed by this means,[8] which when fused recall ordinary aqueous solutions in many respects. Some of them crystallise on solidifying, and then they have, like salts, a definite composition. The property of boric anhydride of forming higher grades of combination with basic oxides when fused explains the power of fused borax to dissolve metallic oxides, and the experiments of Ebelmann on the preparation of artificial crystals of the precious stones by means of boric anhydride. Boric anhydride is, although with difficulty, volatile