SUSPENDED CHANGES IN NATURE 



29 



lias been rmiiislicd hy I'lofessor Cohen of Utrcclit. Tin i- a white 

 crystalline inct.il which does not corrode readily and nnder 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 jmblic 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 show^ed that tin exists in two forms, 

 white tin with a specific gravity of 7.28 and 

 gray tin with a specific gravity of 5.T9. 

 These two forms can be transformed into 

 each other, tlie tiansition temperature 

 being 18°: 



At 18° tin (white) ^ tin (gray). 



But if gray tin is stable lielow 18° how 

 can we explain the fact that tin pails and tin 

 pans remain bright j'ear after year? The 

 average temperature of the northern part of 

 the United States is far below 18°, conse- 

 quently 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 



Fig. 9. The Spread of 

 THE Tix Disease on a Piece 

 OF Sheet Tin. 



the gray tin 

 let the trans- 

 formation of the tin begin, and the spread of the disease is certain. The 

 surface of the tin becomes disfigured with blotches winch gradually 



