BORAX, or BI-BORATE OF SODA. (FB., Borax, borate de soude ; Gm., Borax, Borsauresnatron.) The word Borax forms a convenient heading for this article, as the term is universally lmown ; but, having regard to the fact that various other borates, as well as native boracic acid itself, are utilized either in the production of, or as substitutes for, borax, it is felt that a description of each will be welcomed, especially as the subject has been but very narrowly treated of hitherto.
First of borax. This salt is an acid or bi-borate of sada, Na21340„ forming transpannt crystals of the rhombic system, whose surface becomes efflorescent on exposure to the atmosphere. Its sp. gr. is 1-705 ; it is soluble in water, giving it a weak alkaline reaction. When heated, it melts in its water of crystallization (ten molecules), which is given off, leaving a spongy mass of "burnt borax." When an aqueous solution of borax, sp. gr. 1.245, is allowed to crystallize at about 79° (174° F.), octahedral crystals are obtained, combined with only five molecules of water. This latter is k-nown as " octahedral " borax ; the former, as " prismatic " borax. Octahedral borax crystals are commonly said to be stable only at a relatively high temperature; but Gerney has found that both the prismatic and octahedral forms can be produced at a low temperature, and that 56° (133° F.), which has been indicated as the inferior limit for the production of prismatic borax, is in reality only a temperature near the higher limit at which the production of prismatic borax has been observed, since this salt loses a part of its water at this temperature. Borax absorbs hydrochloric and sulphurous acid gases, and is decomposed by sulphuric, hydrochloric, and nitric acids. Melted borax dissolves almost all metallic oxides, whence its greatest industrial utility. It fills the most important place in the list of boracic compounds, and contains:— Its manufacture will be described in due course. In a natural state, it exists in great abundance in California and in Thihet, being locally known in the latter country as link& (anglice " lineal").
Second ou the list in point of importance, is native boric (boracic) acid (Fa. acide borique, boracique ; GER. Borsiitsre, Boraxsbiure), H„B08, contaioing boracic acid 56.38, water 43-62 per cent. It forms white, scaly, shining crystals, which yield all their water on beating, melting to a transparent mass, which, on cooling, solidifies to a colourless glass. For solution, it requires 25.6 parts of water at 15° (59° F.) ; but only 2.9 parts at 100° (212° F.). Its aqueous solution has only a very slight acid taste ; it colours blue litmus, purple ; and turmeric, reddish brown. Boric anhydride expels almost all other acids from their compounds at a red heat. The greatest natural source of this acid is found in the well-known lagoni of Tuscany ; but it is a common product of volcanic action, and forms a constituent of both sub-akrial and sub-marine hot-spring waters in many parts of the globe.
Next worthy of attention are the variously named and as variously constituted borates of lime, and double borates of lime and soda, known as borocalcite, boronatrocalcite, tincalzite, ulexite, Hayescine, &c. They occur in reuiform masses, varying in size from that of a hazel nut to that of a potato. Their outer crust is hard and chalky looking ; but they are easily broken, and inwardly present a mass of crystalline needles, intersecting each other in all directions, and of brilliant white satiny appearance. These borates are generally associated with reddish or yellowish crystals of glauberite or gypsum, as well as with a certain propoition of common salt, which imparts a brackish flavour. The whole mass is almost completely soluble in strong acids ; but is very slightly soluble in water. These minerals are of such variable composition that no formula can express it with any degree of accuracy. The approximate percentages of the principal ingredients of a number of samples are thus variously stated:— The varieties of this mineral occur principally in the nitrate of soda beds of South America.