The phenomenon of 鈥榖lue flashes鈥� which Celia Jeffery refers to (Letters,
3 April), is a form of crystalloluminescence.
This works best with the 鈥榮elf-sealing鈥� type of envelope, and is quite
easily seen in dim lighting.
When the envelope is sealed a semi-ordered structure forms between the
gum molecules. As this structure is cleaved excess energy is dissipated
in the form of light from excited molecules.
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Similar effects can also be achieved by crushing certain crystalline
solids (try ordinary sugar), although this doesn鈥檛 work as well for the
naked eye.
Crystalloluminescence is closely related to triboluminescence, where
frictional forces are involved in exciting molecules to fluoresce.
Many similar phenomena require sensitive equipment for their observation;
there are in fact millions of light-emitting reactions occurring around
us each second. Some are visible: the majority are not.
In every case, however, the light produced is 鈥榗ool鈥� 鈥� without heat
and so it is unlikely that Jeffery will be able to harness this to dispose
of her mail.
Andrew Alexander University of Edinburgh
