poses at about 6
deg. C., but the dry substance must be heated to about 180 deg.,
before decomposition sets in (see L. Moser, _Abst. J.C.S._, 1907, ii.
p. 549).

Cuprous hydride, (CuH)n, was first obtained by Wurtz in 1844, who
treated a solution of copper sulphate with hypophosphorous acid, at a
temperature not exceeding 70 deg. C. According to E. J. Bartlett and
W. H. Merrill, it decomposes when heated, and gives cupric hydride,
CuH2, as a reddish-brown spongy mass, which turns to a chocolate
colour on exposure. It is a strong reducing agent.

Cuprous fluoride, CuF, is a ruby-red crystalline mass, formed by
heating cuprous chloride in an atmosphere of hydrofluoric acid at 1100
deg.-1200 deg. C. It is soluble in boiling hydrochloric acid, but it
is not reprecipitated by water, as is the case with cuprous chloride.
Cupric fluoride, CuF2, is obtained by dissolving cupric oxide in
hydrofluoric acid. The hydrated form, (CuF2, 2H2O, 5HF), is obtained
as blue crystals, sparingly soluble in cold water; when heated to 100
deg. C. it gives the compound CuF(OH), which, when heated with
ammonium fluoride in a current of carbon dioxide, gives anhydrous
copper fluoride as a white powder.

Cuprous chloride, CuCl or Cu2Cl2, was obtained by Robert Boyle by
heating copper with mercuric chloride. It is also obtained by burning
the metal in chlorine, by heating copper and cupric oxide with
hydrochloric acid, or copper and cupric chloride with hydrochloric
acid. It dissolves in the excess of acid, and is precipitated as a
white crystalline powder on the addition of water. It melts at below
red heat to a brown mass, and its vapour density at both red and white
heat corresponds to the formula Cu2Cl2. It turns dirty violet on
exposure to air and light; in moist air it absorbs oxygen and forms an
oxychloride. Its solution in hydrochloric acid readily absorbs carbon
monoxide and acetylene; hence it finds application in gas analysis.
Its solution in ammonia is at first colourless, but rapidly turns
blue, owing to oxidation. This solution absorbs acetylene with the
precipitation of red cuprous acetylide, Cu2C2, a very explosive
compound. Cupric chloride, CuCl2, is obtained by burning copper in an
excess of chlorine, or by heating the hydrated chloride, obtained by
dissolving the metal or cupric oxide in an excess of hydrochloric
acid. It is a brown deliquescent powder,