GENERAL OPERATIONS

25

Colloidal precipitates, such as aluminium hydroxide, manganese
sulphide, etc., do not obey the law of solubilities referred to above.
Size of Crystals of Precipitates.—Other conditions being
unchanged, it may be said in general that slow precipitation
results in the formation of relatively large crystals and conversely.
The rule that the precipitating reagent should be added slowly
and with continued stirring is a consequence of this fact. But
if it has been found impossible to pro-
duce a precipitate of sufficient coarse-
ness to permit retention by the filter
paper this fault may usually be reme-
died by warming the solution and
precipitate for some time. The actual
result is the resolution of small crystals
and the reprecipitation of their sub-
stance upon the larger ones. This
is due to the fact that very .small
particles have a slightly greater solu-
bility than larger ones. The process of
heating a solution with its precipitate
in this manner is called"digestion.77
Filtration.—After a precipitate has
been separated by filtration and
washed, it is either dried to constant
weight or strongly heated ("ignited77)
in a crucible, in order to bring about
some definite change in its composi-
tion before weighing. In the former
case it is practically necessary to use
a filter of inorganic material because FIG. 5.—Gooch or aiundum
paper cannot be dried to any constant crucible with rubber filtering
1 T* , . ... ring in funnel. (In section.)
degree of hydration. If strong ignition
is to be employed, either paper or inorganic materials may be used
unless burning organic matter exerts a reducing action upon the
precipitate, in which case the use of paper filters is again excluded.
Filter Paper.—For quantitative purposes a paper of very
high grade is required. The texture must be close and uniform
and the material as free as possible from inorganic matter