28 



TTIJ'] 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). 



Fu; 



Okgan l'iri:s that iia\x been Attacked bv tui; Tin Disease. 



Now just as tlic 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- 

 stablc state analogous to the undercooled water. And as tiie addition of 

 a fragment of ice causes undercooled w^ater to solidify, so the addition of 

 a crystal of rhombic snlpliur accelerates the change of monoclinic sul- 

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

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

 stable liquids. This is not surprising wlien wo consider how much more 

 inert solids are tlian lifpiids, especially wlion considered from a chrm- 

 ical standpoint. 



A most iiiterostiiig exnmplo of the retarded transformation of metals 



