Telluric Acid (TeO^{3}) forms six-sided prismatic crystals.  It has not an acid, but rather a metallic taste.  It changes blue litmus paper to red; is slowly soluble in water, and rather sparingly.  Exposed to a high temperature, but not until glowing, the crystalline acid loses its water, and acquires an orange color, but still it preserves its crystalline form, although no longer soluble in water, and is in fact so much changed in its properties as to present the instance of an isomeric modification.

If telluric acid is heated gently in a closed tube, it loses water and turns yellow.  Heated still more strongly, it becomes milk-white, oxygen is expelled, and it is converted into tellurous acid.  The presence of oxygen can be recognized by the more lively combustion which an ignited splinter of wood undergoes when held in it.  Telluric acid produces the same reactions with the blowpipe reagents as tellurous acid.

SEVENTH GROUP.—­LEAD, BISMUTH, TIN.

The oxides of these metals are also reduced to the metallic state by fusion with soda upon charcoal in the flame of reduction, but they are volatilized only after a continued blast, and a sublimate is thrown upon the charcoal.

(a.) Lead (Pb).—­This metal occurs in considerable quantity in nature, chiefly as galena or lead-glance (sulphide of lead).  Likewise, but more rarely, as a carbonate; also as a sulphate, and sometimes combined with other acids and metals.

In the metallic state, lead is of a bluish-grey color, high lustre, and sp. gr. 11.4.  It is soft, and communicates a stain to paper.  It is malleable, ductile, but has very little tenacity.  It melts at about 612 deg..  Exposed to the air it soon tarnishes, being covered with a grey matter, which some regard as a suboxide (Pb^{2}O), and others as simply a mixture of lead and protoxide.  At a glowing heat it is oxidized to a protoxide, and at a white heat it is volatilized.  It is insoluble in most acids.  It is, however, soluble in nitric acid, but without decomposing water.

(L.) Protoxide of Lead (PbO).—­It is an orange-colored powder, which melts at a glowing temperature, and forms a lamellar mass after cooling.  Protoxide of lead absorbs oxygen from the atmosphere while melting, which is given off again by cooling.  Being exposed for a longer while to the air, it absorbs carbonic acid and water, and becomes white on the surface.  It is soluble in nitric acid and caustic alkalies.  It forms with most acids insoluble salts.  It is slightly soluble in pure water, but not in water which contains alkaline salts.  This hydrate is white.

([beta].) Red Oxide of Lead (PbO^{2}, PbO).—­It forms a puce-colored powder.  It is insoluble in caustic alkalies.  Hydrochloric acid dissolves it and forms a yellow liquid, which is soon decomposed into chloride of lead and chlorine.  It is reduced by ignition to the protoxide.