(SKXERAL Ul'KltA77O.V.S*

accomplished by using several small portions of wanh water,
rather than fewer and larger portions,

Drying.............If a precipitate is to be subjected to strong ignition

it is not usually necessary to carry out any preliminary drying,
other than such as may be performed in tin* crucible over a low
flame. But- some precipitates are to be weighed, after drying
at a definite1 temperaf tire, in which eases u drying oven having a
fairly close temperature regulation must be provided. For thin
purpose the use of electrically heated ovens having automatic*
temperature regulation is now
almost universal. Any oven
must have provision for con-
tinuousdisplacemcnt of humidi-
fied air by drier air. Passing t he
entering air through a drying
agent, such us calcium chloride
or sulphuric acid, will facilitate
the drying operaf ion but tltis is
done only in special cases.

In order to understand the
principle of drying it is well to
recall the law that any moist
substance will continue to low
moisture by evaporation until
a certain definite* pressure of
water vapor ("aqueous tension"; is established in ffje Minuititd
ing space*, the value of this pressure depending upon «>ii lite
nature of the moist substance and (ft) the temjHTHture. If I hi*
pressure of the surrounding vapor in reduced bv
moans, evaporation proceeds until equilibrium in
linhed. Thiw by continuing to reduce the external vapor pri«H-
sure, evaporation may be continued. However, it in important
to note that the vapor pressure to be eoriHidfwd m not the lota)
ptmsurc (Hitch im that of thfahnoMphcn*) but the partial |ir«'w«iir<<
of the* vapor of water. AH lite latter priwtw i« directlv pt'iipnr*
tiorial f.o the concentration of wafer vafnir in tin* Mirrotifjfijfi#
•space, the name result will finally be produced (Vi) by rriiurtiitf
the total presHure by means of a pump, f/>; by rf»fitjiiuf»u^lv ilr-.-,.
placing the. nioiht air by itu'iins of some other dried w> nt vr-1 flV

f