has been furnished by Professor Cohen of Utrecht. Tin is a white crystalline metal which does not corrode readily and under ordinary conditions appeal s very permanent. After a particularly cold winter in one of the small towns of northern Germany, it was noticed that in one of the churches the tin pipes of the organ were full of holes and that the tin around the edges of those holes was brittle and would crumble to powder very easily.
A similar occurrence had been reported in St. Petersburg where, after a severe winter, blocks of tin which had been stored in the custom house were found to have crumbled to powder and a number of cases of tin buttons used for military uniforms had undergone a similar change. It was noticed that in the case of the organ pipes, and also on the tin roofs of certain public buildings, the tin had taken on in spots a wartlike appearance; moreover that the warty growth seemed to have the power of spreading. Wherever the tin had changed in this way it had lost its original properties and would easily crumble to gray powder. Because of the appearance of the tin and the spread of the warts over its surface this phenomenon was called the "tin pest" or the "tin disease." That the powder found was still tin and not a product of the corrosion of the metal was easily demonstrated, but the transition of a bright malleable metal to a dull gray powder was for many years a great mystery. But just as there are two kinds of sulphur which can be transformed into each other, so Professor Cohen showed that tin exists in two forms, white tin with a specific gravity of 7.28 and gray tin with a specific gravity of 5.79 
Fig. 9. The Spread of the Tin Disease on a Piece of Sheet Tin. These two forms can be transformed into each other, the transition temperature being 18°:
At 18° tin (white) ⇄ tin (gray).
But if gray tin is stable below 18° how can we explain the fact that tin pails and tin pans remain bright year after year? The average temperature of the northern part of the United States is far below 18°, consequently why do not our tin utensils crumble into the gray modification of the metal? Fortunately for the housewife, the white tin exhibits to a marked degree the property of metastability. It remains unchanged at temperatures far below 18° and even contact with changes it but slowly to the stable form. But once the gray tin let the transformation of the tin begin, and the spread of the disease is certain. The surface of the tin becomes disfigured with blotches winch gradually
