(c.) Tin (Sn).—­This metal does not occur in nature in the metallic state, very seldom in the sulphide, but chiefly in the oxide (tinstone).  In the metallic state it is silver-white, possesses a very high lustre, is soft (but harder than lead), ductile, but has not much tenacity, and it is very malleable.  The metal when it is cast gives a peculiar creaking noise when twisted or bent, which proceeds from the crystalline structure of the metal.  This crystallization is quite clearly manifested by attacking the surface of the metal, or that of tin plate, with acids.

Tin is very slightly tarnished by exposure to the air.  It fuses at 442 deg., and becomes grey, being a mixture of the oxide and the metal.  At a high temperature even, tin is but little subject to pass off as vapor.  It is soluble in aqua regia, and with the liberation of hydrogen, in hot sulphuric and hydrochloric acids, and in cold dilute nitric acid, without decomposing water, or the production of a gas, while nitrate of tin and nitrate of ammonia are formed.  Concentrated nitric acid converts tin into insoluble tin acids.

([alpha].) Protoxide of Tin (SnO) is a dark-grey powder.  Its hydrate is white, and is soluble in caustic alkalies.  When this solution is heated, anhydrous crystalline black protoxide is separated.  The soluble neutral salts of tin-protoxide are decomposed by the addition of water, and converted into acid soluble, and basic insoluble salts.

When protoxide of tin is ignited with free access of air, it takes fire and is converted with considerable intensity into the acids, producing white vapors.  This is likewise the case if it is touched by a spark of fire from steel.  The hydrate of the protoxide of tin can be ignited by the flame of a candle, and glows like tinder.

([beta].) Sesquioxide of Tin (Sn^{2}O^{3}) is a greyish-brown powder.  Its hydrate is white, with a yellow tinge.  It is soluble in aqua ammonia and in hydrochloric acid; this solution forms with solution of gold the “purple of Cassius.”

([gamma].) Stannic Acid (peroxide, SnO^{2}).—­This acid occurs in nature crystallized in quadro-octahedrons, of a brown or an intense black color, and of great hardness (tinstone).  Artificially prepared, it is a white or yellowish-white powder.  It exists in two distinct or isomeric modifications, one of which is insoluble in acids (natural tin-acid) while the other (tin-acid prepared in the wet way) is soluble in acids.  By ignition the soluble acid is converted into the insoluble.  Both modifications form hydrates.