Metals interact with one another in the molten state, as a rule. But metals do not always form chemical compounds with each other when fused. Sometimes one metal simply dissolves in the other. The result is a homogeneous mixture of indefinite composition which cannot be expressed by any distinct chemical formula. Such a mixture is called a solid solution.

   Alloys are legion; nobody has ever taken the trouble to count up even approximately how many of them are known already and how many can be obtained, in general. As in the case of organic compounds, this figure would probably also run into the millions.

   There are some alloys which consist of no less than a dozen metals, and each new addition has a specific effect on their properties. There are many alloys which contain only two metals, these being called bimetallic, but their properties depend on the proportion of their components. Some metals fuse very readily and in any proportion. Such are bronze and brass (an alloy of copper and zinc). Others, such as copper and tungsten, are reluctant to mix under any conditions. Still, scientists have succeeded in making an alloy of them, though in an unusual manner, by what is known as powder metallurgy, that is, by sintering copper and tungsten powders under pressure.

   Some alloys are liquids at room temperature; others are very resistant to high temperatures. The latter are used in large quantities in space engineering. Finally, there are alloys which do not yield even when attacked by the strongest chemical agents, and alloys which are almost as hard as diamond.

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