owever, become so valuable that it is
liable to theft unless constantly under the protection of the user. As
constant protection is often difficult in instructional laboratories,
it is advisable, in order to avoid serious monetary losses, to use
porcelain or silica crucibles whenever these will give satisfactory
service. When platinum utensils are used the danger of theft should
always be kept in mind.

PREPARATION OF CRUCIBLES FOR USE

All crucibles, of whatever material, must always be cleaned, ignited
and allowed to cool in a desiccator before weighing, since all bodies
exposed to the air condense on their surfaces a layer of moisture
which increases their weight. The amount and weight of this moisture
varies with the humidity of the atmosphere, and the latter may change
from hour to hour. The air in the desiccator (see above) is kept at
a constant and low humidity by the drying agent which it contains.
Bodies which remain in a desiccator for a sufficient time (usually
20-30 minutes) retain, therefore, on their surfaces a constant weight
of moisture which is the same day after day, thus insuring constant
conditions.

Hot objects, such as ignited crucibles, should be allowed to cool in
the air until, when held near the skin, but little heat is noticeable.
If this precaution is not taken, the air within the desiccator is
strongly heated and expands before the desiccator is covered. As the
temperature falls, the air contracts, causing a reduction of air
pressure within the covered vessel. When the cover is removed (which
is often rendered difficult) the inrush of air from the outside may
sweep light particles out of a crucible, thus ruining an entire
analysis.

Constant heating of platinum causes a slight crystallization of the
surface which, if not removed, penetrates into the crucible. Gentle
polishing of the surface destroys the crystalline structure and
prevents further damage. If sea sand is used for this purpose, great
care is necessary to keep it from the desk, since beakers are easily
scratched by it, and subsequently crack on heating.

Platinum crucibles stained in use may often be cleaned by the fusion
in them of potassium or sodium acid sulphate, or by heating with
ammonium chloride. If the former is used, care should be taken not
to heat so strongly as to expel all of the sulphuric acid, since the
normal sulphates sometimes expand so rapidly on cooling as to split
the crucible. The fused material sh