The Romance of Modern Chemistry

FROM SOLUTIONS TO CRYSTALS crystallises out from the fused alloy. The difference is that in the alloy we cannot see the crystals which were formed first, because the alloy as a whole has subsequently solidified. In fact, an alloy which has crystallised and has then been cooled down to the ordinary temperature is similar to what a salt solution would become if it were cooled a long way below the freezing-point of water. In the latter circumstances we should not be able to see the salt crystals because of the masses of ice with which they were surrounded. In spite of this difficulty, there are ways and means of finding what sort and shape of crystals have primarily separated from an alloy. In a few cases the Röntgen rays are serviceable, by virtue of the fact that metals differ in their permeability to these rays; some metals are transparent to the rays, others are opaque. Suppose, for instance, we had a small quantity of gold, which is opaque, dissolved in a large quantity of sodium, a metal which is comparatively transparent to the Röntgen rays. Such an alloy if fused would be comparable with a dilute solution of a salt in water. In this latter case, since the solution is dilute, the primary crystallisation on cooling would consist of ice; similarly, the fused alloy, if cooled, would deposit sodium. That is, the first crystals to separate from the alloy would consist of the metal which is transparent to the Rontgen rays, while the spaces between these crystals would ultimately contain the gold, which crystallises last. The slower the cool- ing, the better will be the opportunity for the primary crystals to grow large. If, now, a thin section of the cold alloy is placed on the top of a light-tight envelope containing a sensitive plate, and exposed to the Röntgen rays, these are able to pass 323