[16] Fluoboric acid contains boron fluoride and water, hydrofluoboric acid, boron fluoride, and hydrofluoric acid. It is evident that on the one side the competition between water and hydrofluoric acid, and, on the other hand, their power to combine, are among the forces which act here. From the fact that hydroborofluoric acid, HBF_{4}, can only exist in an aqueous solution, it must be assumed that it forms a somewhat stable system only in the presence of 3H_{2}O. Nothing of this kind occurs with boron chloride, because hydrochloric acid does not act on boric acid. However, amorphous boron at 400° burns in chlorine, and at 410° forms _boron chloride_, BCl_{3}. The boron burns in the chlorine, forming a gas which, in a freezing mixture, condenses into a liquid boiling at 17°, and gives up its excess of chlorine, if there be any, to mercury. The specific gravity of this liquid is 1·42 at 6°. Boron chloride may also be directly obtained from boric anhydride by the simultaneous action of charcoal and chlorine at a high temperature: B_{2}O_{3} + 3C + 3Cl_{2} = 2BCl_{3} + 3CO. It is also obtained by the action of phosphoric chloride on boric anhydride in a closed tube at 200° It is completely decomposed by water, like the chloranhydride of an acid, boric acid being formed; hence it fumes in the air: 2BCl_{3} + 6H_{2}O = 2BH_{3}O_{3} + 6HCl. Boron forms with bromine a similar compound, BBr_{3}, specific gravity at 6° = 2·64, boiling at 90°. The vapour densities of the fluoride, chloride, and bromide of boron show that they contain three atoms of the halogen in the molecule--that is, that boron is a trivalent element forming BX_{3}.[16 bis] [16 bis] Iodide of boron, BI_{3}, was obtained by Moissan (1891), by heating a mixture of the vapours of HI and BCl_{3} in a tube, or by the action of iodine vapour (at 750°) or HI upon amorphous boron. BI_{3} is a solid substance which dissolves in benzol and CS_{2}, reacts with water, melts at 43°, boils at 210°, has a density 3·3 at 50°, and partially decomposes in the light. Besson (1891) obtained BIBr_{2} (boiling at 125°), and BI_{2}Br (boiling at 180°) by heating (300-400°) a mixture of the vapours of HI and BBr_{3}, and showed that NH_{3} combines with BBr_{3} and BI_{3} in various proportions.