The Romance of Modern Chemistry

TWO METALS BETTER THAN ONE copper.” This metal in the pure condition, however, would be too soft for the wear and tear which a coin has to undergo, and consequently 5 per cent, of foreign metal (mostly tin) is added to the pure copper in order to harden it. The curious thing is that tin itself is quite a soft metal, and yet the addition of 5 per cent, of it to pure copper produces an alloy which is much harder than copper itself. Similarly, pure gold and silver are too soft to be used either for coins or ornaments, and for this purpose they must be hardened by the addition of anothci metal, generally copper. The colour of an English silver coin does not betray the presence of copper, but if a three- penny bit were dissolved in nitric acid we should get the blue solution which is characteristic of copper. Since pure silver dissolved in nitric acid gives a colourless solution, we may conclude that our silver coins contain copper ; as a matter of fact, they have per cent, of that metal. An English gold coin is also alloyed with copper to the extent of 1 part for every 11 parts of gold. Such an alloy is described as 22 carat gold, because out of 24 parts of the alloy, 22 parts are pure gold. For ornaments other alloys are made containing less of the precious metal, and described as 18, 12, or 9 cai’at gold, these figures indicating that 24 parts of the alloy contain 18, 12, or 9 parts of pure gold respectively. Another property which is altered by adding a second metal is the fusibility, and an alloy as a rule melts at a much lower temperature than one would expect from the melting-points of the constituent metals. The influence of a foreign body on the melting-point of a pure sub- stance has already been referred to in connection with cast 75