The change by which water is transferred to ice and its reverse may be represented as follows:
At 0° ice ⇄ water.
The arrows pointing in opposite directions indicate that the process is reversible. The significance may be expressed as follows: at 0° ice can be changed to water or water to ice. Similarly in the case of the sulphur.
At 96° sulphur (monoclinic) ⇄ sulphur (rhombic)
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Now just as the water at 0° does not always change to ice, but may be undercooled, so the transformation of monoclinic to rhombic sulphur does not take place immediately, but the needles can exist in a metastable state analogous to the undercooled water. And as the addition of a fragment of ice causes undercooled water to solidify, so the addition of a crystal of rhombic sulphur accelerates the change of monoclinic sulphur to its stable form. The rapidity of the transformation of the metastable solid, however, is by no means as rapid as the change of metastable liquids. This is not surprising when we consider how much more inert solids are than liquids, especially when considered from a chemical standpoint.
A most interesting example of the retarded transformation of metals
