The Romance of Modern Chemistry

METALS, COMMON AND UNCOMMON solidified by the application of cold. Indeed, during winter in extremely cold countries like Siberia the mercury in the bulbs of the thermometers may be frozen; this happens when the temperature falls 40° below zero Fahrenheit. Metals exhibit great variety of density, and it must not be supposed that a metal is necessarily a heavy sub- stance. It is true that mercury is nearly fourteen times as heavy as water, and that gold is about nineteen times as heavy, yet there are metals—sodium, for instance—which are lighter than water ; if a piece of this metal is thrown on water it swims about on the surface. The reader may remember aluminium as a comparatively light metal, the weight of which, bulk for bulk, is only one-seventh of that of gold. Only a few of the metals are found in the uncombined or “ native ” condition. These are the so-called “ noble ” metals—gold, platinum, &c.—which are distinguished by the fact that they do not tarnish, and are not readily attacked by acids. Other metals occur in the form of ores, and have to be extracted from these by laborious processes. However it has come about, the majority of the metals have combined with oxygen or sulphur, and their ores consist therefore mainly of oxides or sulphides, mixed naturally with a smaller or greater amount of earthy matter. The operations or metallurgical processes necessary for winning metals from their ores are modified by the idiosyncrasies of the particular metal which is sought, but the essential chemical reaction involved is generally the removal of oxygen from the ore by the agency of carbon. If the ore does not already consist of the oxide, the latter is obtained by roasting the sulphide in a current of air, by which process the sulphur in the sulphide is replaced by oxygen. 62