225. Amalgams.—An amalgam is an alloy of Hg and another metal. Mirrors are "silvered" with an amalgam of Sn. Tin-foil is spread on a smooth surface and covered with Hg, and the glass is pressed thereon.
Various amalgams are employed for filling teeth, a common one being composed of Hg, Ag, and Sn. Au or Ag, with Hg, forms an amalgam used for plating. Articles of gold and silver should never be brought in contact with Hg. If a thin amalgam cover the surface of a gold ring or coin, Hg can be removed with HNO3, as Au is not attacked by it. Would this acid do in case of silver amalgam? Heat will also quickly cause Hg to evaporate from Au.
CHAPTER XLIV.
SODIUM AND ITS COMPOUNDS.
Examine NaCl, Na2SO4, Na2CO3, Na, NaOH, HNaCO3, NaNO3.
226. Order of Derivation.—Though K is more metallic, or electro- positive, than Na, the compounds of Na are more important, and will be considered first. The only two compounds of Na which occur extensively in nature are NaCl and NaNO3. Almost all others are obtained from NaCl, as shown by this table, which should be memorized and frequently recalled.
) Na
NaCl ) Na2SO4) Na2CO3) NaOH
NaNO3) ) ) HNaCO3
From what is Na2SO4 prepared, as shown by the table? Na2CO3? Na?
227. Occurrence and Preparation of NaCl.—NaCl occurs in sea water, of which it constitutes about three per cent, in salt lakes, whose waters sometimes hold thirty per cent, or are nearly saturated, and, as rock salt, in large masses underground. Poland has a salt area of 10,000 square miles, in some parts of which the pure transparent rock salt is a quarter of a mile thick. In Spain there is a mountain of salt five hundred feet high and three miles in circumference. France obtains much salt from sea water. At high tide it flows into shallow basins, from which the sun evaporates the water, leaving NaCl to crystallize. In Norway it is separated by freezing water, and in Poland it is mined like coal. In New York and Michigan it is obtained by evaporating the brine of salt wells, either by air and the sun's heat, or by fire. Slow evaporation gives large crystals; rapid, small ones.
228. Uses.—The main uses are for domestic purposes and for making the Na and Cl compounds. In the United States the consumption amounts to more than forty pounds per year for every person.