16 THE ELEMENTS.
Yield 40 to 45 g.; theoretically 46.4 g. This process is used
technically for recovering lead from litharge formed in cupel-
lation.
1Density Determination. Fasten a clean piece of lead weighing
from 5 to 10 g. in the loop of a hair, or silk thread. Determine
the weight of the lead in air (m), and the loss in weight (w) when
it is entirely submerged in water; then if Q is the density of water
at the temperature of the experiment
2
and X = 0.0012 the density
of air, then d, the density of the lead, is given by the following
equation:The density should be computed to not more than five significant
figures. The observations should be checked with the same
piece of lead, or with different pieces from the same preparation.
The density of lead is about 11.351.
The density of an element varies somewhat with the method of
preparation. Thus with antimony distilled in vacuum, d = 6.62;
pressed antimony, d = 6.69; gold distilled, d = 18.88, pressed at
10,000 atmospheres, d = 19.27fr.
ALUMINOTHERMY.
Metallic aluminium resists the action of water and the atmosphere, not
because the metal is difficult to oxidize, but on account of the fact that its
surface becomes covered with a thin, coherent layer of oxide which protects
it from further attack. If the formation of this layer of oxide upon the
aluminium is prevented by amalgamation, then the aluminium is rapidly
attacked. Aluminium vessels disintegrate quickly if they are amalgamated
with even a trace of mercury.
Remove the oil from 2 to 5 g. of aluminium powder by boiling
it with a little alcohol; pour off the latter, and cover the alu-
minium with a |% solution of mercuric chloride. After a few
minutes decant off the liquid and wash the powder several times
with water. Then cover the amalgamated metal with water and
allow it to stand. Within a short time there is an evolution
of hydrogen, the mass becomes heated until finally vapors of
steam arise, and white hydrated aluminium oxide is formed.
(^1) Cf. No. 13.
(^4) Cf. Tables of specific gravity.