Tin-oxides exposed to the oxidation flame, then moistened with a solution of cobalt, and exposed again to the flame of oxidation, will exhibit, after having completely cooled, a bluish-green color.

EIGHTH GROUP.—­MERCURY, ARSENIC.

These two metals are volatilized at a temperature lower than that of a red heat, and produce, therefore, no reactions with borax and microcosmic salt.  Their oxides are easily reduced to the metallic state.

(a.) Mercury (Hg).—­This metal occurs in nature chiefly combined with sulphur as a bisulphide.

It occurs still more rarely in the metallic form, or combined with silver, selenium, or chlorine.

Mercury, in the metallic state, has a strong lustre, and is liquid at ordinary temperatures, whereby it is distinguished from any other metal.  It freezes at 40 deg. and boils at 620 deg., but it evaporates at common temperatures.  Pure mercury is unalterable.  Upon being exposed to the air, it tarnishes only by admixture with other metals, turns grey on the surface, and loses its lustre.  It is soluble in cold nitric acid and in concentrated hot sulphuric acid, but not in hydrochloric acid.

([chi].) Protoxide of Mercury (Hg^{2}O).—­It is a black powder, which is decomposed by ignition into metallic mercury and oxygen.  By digestion with certain acids, and particularly with caustic alkalies, it is converted into metallic mercury and peroxide.  Some neutral salts of the protoxide are only partly soluble in water, as they are converted into basic insoluble and acid soluble salts.

Protoxide of mercury is completely insoluble in hydrochloric acid.  Its neutral salts change blue litmus paper to red.

([beta].) Peroxide of Mercury (HgO).—­This oxide exists in two allotropic modifications.  One is of a brick-red color, and the other is orange.  Being exposed to heat, they turn black, but regain their respective colors upon cooling.  They are decomposed at a high temperature into metallic mercury and oxygen.  They yield with acids their own peculiar salts.

Mercury, in the metallic form, can never be mistaken for any other metal in consequence of its fluid condition at ordinary temperatures.

Exposed to the blowpipe flame, it is instantly volatilized.  This is also the case with it when combined with other metals.  The oxides of mercury are, in the oxidation and reduction flames, instantly reduced and volatilized.  They do not produce any alteration with fluxes, as they are volatilized before the bead melts.  Heated with carbonate of soda in a glass tube closed at one end, they are reduced to metallic mercury, which is volatilized, and condenses upon a cool portion of the tube as a grey powder.  By cautious knocking against the tube, or by rubbing with a glass rod, this sublimate can be brought together into one globule of metallic mercury.  Compounds of mercury can be most completely reduced by a mixture of neutral oxalate of potassa and cyanide of potassium.  If the substance under examination contains such a small quantity of mercury that it cannot be distinguished by volatilization, a strip of gold leaf may be attached to an iron wire, and introduced during the experiment in the glass tube.  The smallest trace of mercury will whiten the gold leaf in spots.