10. Tin.—­This metal is used in the form of foil, cut into strips 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 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.