er, 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
used to detect hydrofluoric acid, which changes its color to yellow.

All the test-papers are to be cut into narrow strips, and preserved in
closely stopped vials. The especial employment of the test-papers we
shall allude to in another place.

B. ESPECIAL REAGENTS.

13. _Fused Boracic Acid_ (BO^{3}).--The commercial article is
sufficiently pure for blowpipe analysis. It is employed in some cases
to detect phosphoric acid, and also minute traces of copper in lead
compounds.

14. _Fluorspar_ (CaFl^{2}).--This substance should be pounded fine and
strongly heated. Fluorspar is often mixed with boracic acid, which
renders it unfit for analytical purposes. Such an admixture can be
detected if it be mixed with bisulphate of potassa, and exposed upon
platinum wire to the interior or blue flame. It is soon fused, the
boracic acid is reduced and evaporated, and by passing through the
external flame it is reoxidized, and colors the flame green. We use
fluorspar mixed with bisulphate of potassa as a test for lithia and
boracic acid in