Boron, the characteristic combustible element of the acid contained in borax. In nature it is always met with in combination with oxygen. It is found in small quantities, and only in a few localities. It presents considerable analogy with silicon in its properties and its mode of combination, and like it may be obtained in two distinct modifications, the crystalline and the amorphous. Berzelius obtained boron by heating the borofluoride of potassium with an equal weight of potassium in a covered iron crucible. Boron as thus obtained is an amorphous, dull olive-green powder, which before it has been strongly ignited soils the fingers, and is dissolved by pure water in small quantity, forming a greenish yellow solution; from which, however, it is precipitated unchanged on adding a little solution of sal ammoniac. Boron is not oxidized by exposure to air, to water, or to solutions of the alkalies, whether cold or boiling. It is, however, easily oxidized when treated with nitric acid or with aqua regia. After exposure to intense heat in vessels from which air is excluded, it becomes denser and darker in color. It may be fused by the application of a heat still more intense than that required to melt silicon.

As first obtained, boron exhibits a strong attraction for oxygen, and, if heated in air or in oxygen, takes fire below redness, burning with a reddish light and emitting vivid scintillations; it is thus converted superficially into boric anhydride, B2O3, which melts and protects a portion of the boron. Mixed with nitre and heated to redness, it deflagrates powerfully. It is also oxidized when ignited with hydrate of potash; and when heated with carbonate of potassium in fusion it sets carbon free, and forms borate of potassium. Pulverulent boron, like silicon, is a non-conductor of electricity. Boron may be obtained in the amorphous form in large quantity by the following method (Wohler and Deville; Liebig's Annalen, cv. 67): 1,500 grains of fused boric anhydride are coarsely powdered and mixed rapidly with 900 grains of sodium cut into small pieces. The mixture is then introduced into a cast-iron crucible previously heated to bright redness; 700 or 800 grains of solid but previously fused chloride of sodium are placed upon the top of the mixture, and the crucible is covered.

As soon as the reaction is over, the still liquid mass is thoroughly stirred with an iron rod, and poured while red hot, in a slender stream, into a large and deep vessel containing water acidulated with hydrochloric acid. The undissolved pulverulent boron is then collected on a filter and washed with acidulated water till the boric acid is got rid of; after which the washing is continued with pure water till the boron begins to run through the filter. It must finally be dried upon a porous slab without the application of heat. - Crystallized Boron. In order to convert the amorphous into the crystallized form, the following method may be adopted: A small Hessian crucible is lined with the pulverulent boron made into a paste with water, and the boron is pressed in strongly, as in the ordinary mode of lining a crucible with charcoal. In the central cavity a piece of aluminum weighing from 60 to 90 grains is placed; the cover is luted on and the crucible enclosed in a second, the interval between the two being filled with recently ignited powdered charcoal. The outer crucible is next closed with a luted cover, and the whole exposed for a couple of hours to a heat sufficient to fuse nickel.

The temperature is then allowed to fall, and when cold the contents of the inner crucible are digested in diluted hydrochloric acid, which dissolves out the aluminum; beautiful crystals of boron are left, generally transparent, but of a dark brown color. Numerous scales of boron are formed at the same time, in pale copper-colored, opaque, six-sided plates, which consist of an alloy of aluminum and boron, formerly erroneously called graphitoid boron. Crystallized boron has a specific gravity of 2.68; it assumes the form of transparent octahedrons belonging to the pyramidal system. These crystals when pure are nearly colorless, but they usually contain traces of foreign matter, which give them a pale yellow or red color; they refract light powerfully, and are hard enough to scratch the ruby, and even sensibly to wear away the diamond. Crystallized boron burns imperfectly in oxygen when heated to full whiteness, and becomes coated with a layer of fused boric anhydride. It however burns easily when heated to redness in dry gaseous chlorine, becoming converted into the gaseous terchloride of boron.

No acid or mixture of acids has any action upon the crystalline boron. - The atomic weight of boron is 10.9. The hardness of boron has suggested its use as a substitute for the diamond in cutting glass, for drills, and bearings of machinery; but the cost of production has hitherto prevented its extensive application.