DRY REACTIONS; FURNACES 17
in pure water or a neutral salt solution. When a solution which
is acid must be rendered exactly neutral, base is added until the
solution gives the neutral tint to litmus. If the contamination
will do no harm, a drop or two of the litmus solution is added
directly to the liquid; otherwise, a drop of the liquid must be with-
drawn on a stirring rod and touched to a piece of litmus paper.
It is a tedious operation exactly to neutralize a solution in this
way, but the process is greatly facilitated if a fraction of the liquid
to be neutralized is held in reserve in another vessel. The reagent
may be added rather freely to the main portion until the neutral
point, is not only reached but overstepped. Then a part of the
reserve, may be added and the reagent again added, but more
cautiously this time, and so on until the whole solution is exactly
neutralized.
The procedure outlined in the last paragraph is a general one
to follow whenever adding a reagent which must be used in exactly
the right amount and not in excess: always hold a fraction of the
original material in reserve before adding the reagent to the main
portion.
- DRY REACTIONS; FURNACES
Dry solid substances do not react appreciably with each other
at ordinary temperature. Reactions are made possible in two
ways: first, the wet way, in which the substances are dissolved and
thus brought into most intimate contact. In many cases solution
also produces ionization, which, as is known, greatly increases
chemical activity.
Reactions in the dry way are rendered possible by heat. Heat
alone increases the rapidity of a chemical reaction, it being a gen-
eral law that the speed is increased from two to three times for
every increase of 10°C. in temperature. If one or more of the
reacting substances are melted by the heat, the same sort of inti-
mate contact is brought about as in solutions. Fusion is like-
wise a means of producing electrolytic dissociation, and on this
account also it increases chemical activity.
In some of the furnace reactions in which none of the substances
are melted, as, for example, in the reduction of strontium sul-
phate to strontium sulphide by means of charcoal (see Prepa-
ration 20), the process probably takes place by virtue of a cer-
tain amount of gas which is continuously regenerated. A little
