28 



THE POPULAR SCIENCE MONTHLY 



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). 



Fig. 8. Organ Pipes that have been Attacked by the Tin Disease. 



Xow 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 meta- 

 stable 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 sul- 

 phur to its stable form. The rapidity of the transformation of the 

 metastable solid, however, is by no means as rapid as the change of meta- 

 stable liquids. This is not surprising when we consider how much more 

 inert solids are than liquids, especially when considered from a chem- 

 ical standpoint. 



A most interesting example of the retarded transformation of metals 



