about half an inch wide. Tin is very susceptible of oxidation, and
therefore deprives oxidized substances of their oxygen very quickly,
when heated in contact with them. It is employed in blowpipe analysis,
for the purpose of producing in glass beads a lower degree of
oxidation, particularly if the substance under examination contains
only a small portion of such oxide. These oxides give a characteristic
color to the bead, and thus are detected. The bead is heated upon
charcoal in the reduction flame, with a small portion of the tin,
whereby some of the tin is melted and mixes with the bead. The bead
should be reduced quickly in the reduction flame, for by continuing
the blast too great a while, the oxide of tin separates the other
oxides in the reduced or metallic state, while we only require that
they shall only be converted into a sub-oxide, in order that its
peculiar color may be recognized in the bead. The addition of too much
tin causes the bead to present an unclean appearance, and prevents
the required reaction.


11. _Silica_ (SiO^{3}).--This acid does not even expel carbonic acid
in the wet way, but in a glowing heat it expels the strongest volatile
acids. In blowpipe analysis, we use it fused with carbonate of soda to
a bead, as a test for sulphuric acid, and in some cases for phosphoric
acid. Also with carbonate of soda and borax, for the purpose of
separating tin from copper.

Finely powdered quartz will answer these purposes. If it cannot be
procured, take well washed white sand and mix it with two parts of
carbonate of soda and two parts of carbonate of potassa. Melt the
materials together, pound up the cooled mass, dissolve in hot water,
filter, add to the filtrate hydrochloric acid, and evaporate to