e desiccator is moved.

Desiccators should never remain uncovered for any length of time. The
dehydrating agents rapidly lose their efficiency on exposure to the
air.

CRUCIBLES

It is often necessary in quantitative analysis to employ fluxes to
bring into solution substances which are not dissolved by acids. The
fluxes in most common use are sodium carbonate and sodium or potassium
acid sulphate. In gravimetric analysis it is usually necessary to
ignite the separated substance after filtration and washing, in order
to remove moisture, or to convert it through physical or chemical
changes into some definite and stable form for weighing. Crucibles
to be used in fusion processes must be made of materials which will
withstand the action of the fluxes employed, and crucibles to be used
for ignitions must be made of material which will not undergo any
permanent change during the ignition, since the initial weight of the
crucible must be deducted from the final weight of the crucible and
product to obtain the weight of the ignited substance. The three
materials which satisfy these conditions, in general, are platinum,
porcelain, and silica.

Platinum crucibles have the advantage that they can be employed at
high temperatures, but, on the other hand, these crucibles can never
be used when there is a possibility of the reduction to the metallic
state of metals like lead, copper, silver, or gold, which would alloy
with and ruin the crucible. When platinum crucibles are used with
compounds of arsenic or phosphorus, special precautions are necessary
to prevent damage. This statement applies to both fusions and
ignitions.

Fusions with sodium carbonate can be made only in platinum, since
porcelain or silica crucibles are attacked by this reagent. Acid
sulphate fusions, which require comparatively low temperatures, can
sometimes be made in platinum, although platinum is slightly attacked
by the flux. Porcelain or silica crucibles may be used with acid
fluxes.

Silica crucibles are less likely to crack on heating than porcelain
crucibles on account of their smaller coefficient of expansion.
Ignition of substances not requiring too high a temperature may be
made in porcelain or silica crucibles.

Iron, nickel or silver crucibles are used in special cases.

In general, platinum crucibles should be used whenever such use is
practicable, and this is the custom in private, research or commercial
laboratories. Platinum has, h