CARBIDES. 97nearest to the source of hydrogen containing about five grams of
red phosphorus and the other four grams of magnesium powder.
The phosphorus must be perfectly dry (cf. p. 85). Erect the
apparatus under the hood.
After filling the apparatus with hydrogen (Test!), heat the tube
slightly through its entire length to expel moisture; then, while
maintaining a steady current of hydrogen, heat both boats as
equally as possible by means of two Bunsen burners so that phos-
phorus vapor is carried over the magnesium and a violent reactiontakes place. When all the phosphorus has volatilized, allow the
contents of the tube to cool in the atmosphere of hydrogen.
Magnesium phosphide when thrown into water is decomposed,
and phosphine is set free. This gas has a very offensive odor;
it is combustible, but it does not, like the impure phosphine that is
commonly prepared, take fire spontaneously.
(/) Carbides.- Calcium Carbide; Acetylene from Calcium Carbide;
Benzene from Acetylene.
Calcium Carbide. Heat 0.5 g. of lampblack strongly in a
porcelain crucible for a few minutes; after it has cooled mix with
it 0.5 g. of thin shavings of metallic calcium, and heat the mixture
strongly with the blast lamp for a few minutes. After cooling
there is found in addition to lampblack a sintered white mass
which contains some calcium carbide; on testing it with water,
acetylene is formed.
Acetylene from Calcium Carbide. Allow water to drop slowly
from a dropping funnel upon a few pieces of commercial calcium
carbide in a half-liter flask. Pass the acetylene evolved through
a solution of sodium plumbite in order to remove any hydrogen
sulphide, and through an acid solution of copper sulphate to take
out any phosphine. Acetylene burns with a brilliantly luminous
and, unless in a special burner, very smoky flame. Before igniting
the gas it should be tested to see whether all the air has been
removed from the apparatus.At the temperature of the electric arc, acetylene can be formed from its
elements with absorption of heat. At a bright red heat it decomposes, but at
lower temperatures, although it is likewise unstable, its rate of decomposition