In certain lakes in California the water, when evaporated, deposits crystals of the formula NagB^^. ioH₂O ; this substance is named borax. It is a white, crystalline salt, easily soluble in hot water, but sparingly soluble in cold. When mixed with sulphuric acid nacreous scales separate of the formula B(OH)₃, or, as it is usual in writing the formulae of acids to place the hydrogen atoms first, H₃BO₃. Boracic acid hardly deserves the name of acid ; in aqueous solution it exists almost entirely in the non-ionised state. No ions are volatile ; but this compound issues in Tuscany and in the Lipari Islands along with steam from cavities in the ground, termed suffioni; it is easily recognised, for it imparts a green colour to a flame held in the steam. When heated to ioo° boracic acid loses water and is changed into metaboracic acid, 0=B-OH, a vitreous substance; and at a red-heat boron oxide, B₂O₃, is left ^as a transparent, colourless glass. Its constitution is 0=B-O-B=0.

The borates of the alkalies are prepared by mixing boracic acid with the hydroxide of the alkali metal; although there are very few hydrogen ions in an aqueous solution of boracic acid, however dilute, yet some of those present combine with the hydroxyl ions of the alkali, forming water, thus: H₃=BO₃.Aq + 3Na-OH.Aq=: Na₃=BO₃.Aq + 3H₂O. But there are so few ions present, that those of the water, which, it will be remembered, is ionised, although to an extremely minute extent, are yet sufficiently numerous to bear some proportion to those of the boracic acid ; hence the reaction given above is perceptibly reversed, and on dissolving borax in water it is " hydrolysed," that is, split by the hydrogen and hydroxyl ions of the water into non-ionised boracic acid and caustic soda, the latter, of course, largely ionised as usual. It is therefore possible to estimate the sodium of borax by addition of a solution of a strong acid, such as hydrochloric or sulphuric acid of known concentration, just as if no boracic acid were present, provided methyl-orange be used as an indicator. (See p. 75.) Thus the addition of 36.5 grams (H=i; 01=35.5) of hydrochloric acid, dissolved in a litre of water (such a solution is termed a "normal solution"), to 191 grams of a solution of crystallised borax in a litre of water (1/2 [Na₂ = 46+ B₄ = 44 + O₇= 112 4- ioH₂O = 180]) ; (in all 1/2 of 382) gives a solution which is neutral to methyl-orange.

Fused borax has the property of dissolving oxides of the metals, forming complex borates; certain of these are coloured, and their formation is often made use of for detecting the presence of such metals as copper (blue), silver (yellow), chromium (green), ferric iron (yellow), ferrous iron (bottle-green), manganese (amethyst, when heated in a flame containing excess of oxygen), cobalt (blue), and nickel (reddish). Borax is also used for soldering easily oxidisable metals, such as iron, copper, or brass ; the film of oxide which prevents the metal touching and alloying with the solder is thus removed. Both borax and boracic acid have considerable antiseptic properties, and are used for preserving eggs, milk, and other animal and vegetable substances.