d if this be decomposed with dilute hydrochloric
acid a very evil-smelling gas, consisting of a mixture of hydrogen and
boron hydride, is obtained. This mixture burns with a green flame
forming boron trioxide; whilst boron is deposited on passing the gas
mixture through a hot tube, or on depressing a cold surface in the gas
flame. By cooling it with liquid air Sir W. Ramsay and H.S. Hatfield
obtained from it a gas of composition B3H3. The mixture probably
contained also some BH3 (W. Ramsay and H.S. Hatfield, _Proc. Chem.
Soc._, 17, p. 152). Boron fluoride BF3 was first prepared in 1808 by
Gay Lussac and L. Thenard and is best obtained by heating a mixture of
the trioxide and fluorspar with concentrated sulphuric acid. It is a
colourless pungent gas which is exceedingly soluble in water. It fumes
strongly in air, and does not attack glass. It rapidly absorbs the
elements of water wherever possible, so that a strip of paper plunged
into the gas is rapidly charred. It does not burn, neither does it
support combustion. A saturated solution of the gas, in water, is a
colourless, oily, strongly fuming liquid which after a time
decomposes, with separation of metaboric acid, leaving hydrofluoboric
acid HF.BF3 in solution. This acid cannot be isolated in the free
condition, but many of its salts are known. Boron fluoride also
combines with ammonia gas, equal volumes of the two gases giving a
white crystalline solid of composition BF3.NH3; with excess of ammonia
gas, colourless liquids BF3.2NH3 and BF3.3NH3 are produced, which on
heating lose ammonia and are converted into the solid form.

Boron chloride BCl3 results when amorphous boron is heated in chlorine
gas, or more readily, on passing a stream of chlorine over a heated
mixture of boron trioxide and charcoal, the volatile product being
condensed in a tube surrounded by a freezing mixture. It is a
colourless fuming liquid boiling at 17-18 deg. C, and is readily
decomposed by water with formation of boric and hydrochloric acids. It
unites readily with ammonia gas forming a white crystalline solid of
composition 2BCl3.3NH3.

Boron bromide BBr3 can be formed by direct union of the two elements,
but is best obtained by the method used for the preparation of the
chloride. It is a colourless fuming liquid boiling at 90.5 deg. C.
With water and with ammonia it undergoes the same reactions as the
chloride. Boro