A section of the salina where fully developed shows the following strata, :— 1. The costra or crust, composed of earthy matters, angular pieces of rock-salt, and other saline matters.
2. Caliche, composed of granular layers of nitrate of soda, containing salt, other saline and earthy substances, and angular pieces of stone, often accompanied by much sulphate of soda and some magnesia-alum, mixed with earthy matters, as silica, alumina, carbonate and sulphate of lime ; also iodine, bromine, chlorine, and the boracic acid mineral locally known as tisa or tiza.
3. Coba, the general loose, earthy covering to the silico-calcareous and porphyritic rocks, in which the borate is found in nodules, from the size of a pea to 2 ft. in diameter ; or in thin stria+, with sulphate of lime.
The best borate-bearing ground is flat, and its surface is free from salt, the tisa making its appear ance iu nodules of all sizes, but generally that of a large potato. They are sparsely or plentifully imbedded in a now dry saline mud, which is, however, damp and even wet in some places from the percolation of water. Much glauberite,, in large and small crystals, is sometimes combined with the borate ; while at other times, the strata is made up entirely of borate. Pure borate is found only in a few places ; that occurring under the salitres is accompanied by glauberit,e.
As has been already remarked, the grade of the borate varies exceedingly, so that no general average composition can be given. This seems to be owing to the fact that where it occurs, are numerous deposits of chloride of calcium and sulphate of lime ; and, though uo rain falls in the district, the deposits retain sufficient moisture to keep them quite soft, like half-dried mud. The constitution of any compound resulting from a mixture of solutions of these salts must necessarily vary with their proportions. There is every probability that the formation of the mineral is due to preeipitation, the after shrinking of such a gelatinous precipitate accounting for the nodular form in which it is found. It is easy to collect specimens with composition varying from almost pure bi-borate of soda to none at all, as in samples 5 and 9 (p. 526), which have evidently been
formed from sulphate of lime, and an indefinite compound of soda with boracic acid, only a small part of the resulting sulphate of soda having drained off. The richest specimens are probably formed, in the opinion of Mr. Walker, from chloride of calcium and a sodio-boracie salt, the resulting chloride of sodium, owing to its solubility at ordinary temperatures, draining off more readily than sulphate of soda under the same conditions.
Stiata of borate of lime, calculated at over 3,000,000 cub. yds. in extent, have been found in the dried-up lake bed of Maricunga, situated to the north-west of Copiapo, on the western slope of the Andes. The lake is situate at 12,000 ft. above the sea, and is entirely shut in by mountains of volcanic formation, abounding in trachyte and pumice. Borat,e of lime, in the form of white silky flakes, has been discovere,d suspended in the waters of the hot springs called BaMa del Toro, in the Cordillera of Coquimbo.
On the authority of Dr. Phipson, it forms an excellent flux for metallurgical purposes, and has been employed with success in the porcelain manufactories of Sevres; it app'ears capable of effec tually replacing borax in all the applications of that salt. The boracic acid may be exlracted from the mineral, by saturating the pulverized substance in boiling dilute hydrochloric acid. The clear solution is decanted while hot, and, upon cooling, the liquid throws down the boracic acid in large quantities.
Test for Boracie Acid.—Macerate a small sample of the suspected mineral in alcohol, adding a little strong sulphuric acid, ignite the solntion, and allow it to evaporate; the merest trace of boracic acid, whether free or combined, will colour the flame green.
Preparation of the Imported Artieles.—The boracic c,ompounde as imported are seldom in a fit stato for use, and are therefore submitted to a preparative process after arrival in this country. The process of course varies with the substance ; thus, the boracic acid of Tuscany has to be con verted into borax; the tincal from Asia needs purification ; and the borates of lime require cleanaiug, or transformation into borate of soda.