dryness. Moisten the dry residue with hydrochloric acid, and boil in
water. The silica remains insoluble. It should be washed well, dried,
and heated, and then reduced to powder.


12. TEST-PAPERS.--(_a._) _Blue, Litmus Paper._--Dissolve one part of
litmus in six or eight parts of water, and filter. Divide the filtrate
into two parts. In one of the parts neutralize the free alkali by
stirring it with a glass rod dipped in diluted sulphuric acid, until
the fluid appears slightly red. Then mix the two parts together, and
draw slips of unsized paper, free from alkali, such as fine filtering
paper. Hang these strips on a line to dry, in the shade and free from
floating dust. If the litmus solution is too light, it will not give
sufficient characteristic indications, and if too dark it is not
sensitive enough. The blue color of the paper should be changed to
red, when brought in contact with a solution containing the minutest
trace of free acid; but it should be recollected that the neutral
salts of the heavy metals produce the same change.

(_b._) _Red Litmus Paper._--The preparation of the red litmus paper is
similar to the above, the acid being added until a red color is
obtained. Reddened litmus paper is a very sensitive reagent for free
alkalies, the carbonates of the alkalies, alkaline earths, sulphides
of the alkalies and of the alkaline earths, and alkaline salts with
weak acids, such as boracic acid. These substances restore the
original blue color of the litmus.

(_c._) _Logwood Paper._--Take bruised logwood, boil it in water,
filter, and proceed as above. Logwood paper is a very delicate test
for free alkalies, which impart a violet tint to it. It is sometimes