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Grey Tin






In 1851 O. L. Erdmann discovered organ-pipes covered with warty excrescences which developed into holes owing to the conversion of the tin into a grey powder. Similar observations have been repeatedly made. Aristotle speaks of tin "melting" at low temperature; and bars of tin kept at Petrograd and Moscow during severe winters have been found covered with a grey powder owing to what has been called "tin-pest"; whilst coins and utensils made of tin, preserved in museums, have developed spots upon their surfaces owing to the contraction of "museum-sickness"! The cause of this behaviour of tin has been investigated by Cohen and van Eijk, who have shown that tin is enantiotropic - that is, it exists in two allotropic modifications which change the one into the other with change of temperature according to the scheme:

grey tin ⇔ white tin.

transformation of white into grey tin
The transformation of white into grey tin.
The transition temperature at which both forms are equally stable lies very near 20° C.; above this temperature ordinary white tin is stable, below it metastable. Consequently when the metal is kept below 20° C. it continuously changes into the grey form. Under ordinary atmospheric conditions the rate of this change is very slow, but it is accelerated by lowering of temperature, reaches its maximum at about - 50° C., and then diminishes again. Fig. sets forth the experimental results of Cohen and van Eijk. The density of grey tin is not known accurately, but it is about 5.8 at 15° C.


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