Alerrnry orifh Ault is • preparatlen used in medicine. It is made Air rubbing chalk and mercury together until the latter is no minutely divided as not to be perceptible by the naked eye It is sometimes ballet gear powebe. and from exposure to light LI very liable to contain protozids and Lincohle of meneury.
Mercury mita magnesia is a preparation similar to that just described. Ths mixture of the two Is facilitated by the introduction of • little manna, which le subsequently got rid of by washing with water.
Other preparations containing finely divided metallic mercury are also used in medicine. Such are the pill, ointment, and cerato of mercury.
Tests for mereery. The salts of mercury are distinguished by being volstiliaed by heat, and by yielding globules of metal when heated with carbonate of el& in a narrow test tube. From solution the metal is precipitated with its characteristic appearance on to a piece of gold (such as a half-sovereign) when the liquid is rubbed on the gold with a key or other piece of steel. Metallic copper also causes the deposi tion of mercury from solution with oharacteristic lustre. Protochloride
of tin has the rune effect.. Sulphuretted hydrogen throws down a black precipitate.
Proto- or mercurous salts are distinguished by turning black on the addition of an alkali. From solution, hydrochloric acid throws down a white precipitate (calomel), and iodide of potassium a green pre cipitate.
Per- or bi- or mercuric salts give no precipitate with hydrochloric soid, a yellow (binoxide) with fixed alkalies, white with ammonia, and scarlet with iodide of potassium.
Estimation of mercury. This is usually effected in the metallic form. It is brought to this state either by protochloride of tin, or by heating with quick limo in a glass tube through which a current of hydrogen is passing, the mercury being collected in a glass bulb.
Sometimes mercury is precipitated by aulphuretted hydrogen, and estimated in the state of bisulphide. In that case it is always advisable to wash the sulphide with sulphite of soda solution, which hoe the effect of dissolving any free sulphur.