> Always wondered, in the case of the whole "heat pack" thing though if
> you couldn't make one that did the reverse, and was "recoolable", or
> whatever.

Hmm. Plausibly.

I would imagine, like with other refrigeration technologies, the key is 
to find a material which has a phase change at the right temperature. 
Normal hand warmers become super-saturated solutions at very high 
temperatures. If you wanted to make something cold, you'd need something 
that becomes super-saturated at a low temperature.

I'm also not completely sure of the kinetics: a hand warmer releases 
heat quickly, and absorbs it slowly. That may or may not be useful for 
cooling purposes.

Unrelated, but I note that dissolving ammonia in water is an endothermic 
reaction. That, apparently, is how Victorian icecream was made. So I 
guess the question is, WOULD YOU EAT IT? ;-)

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