A substance will both heat up and cool down at the same rate.  You can't
get a rapid heat absorbtion and a slow heat release, at least not out of
simple substances without a chemical reaction.  Of course, the difference
in temp makes a difference, ie if you heat it with a heat source that is
100 degrees hotter than the rock, then let it release its heat to an
atmosphere that is only 50 degrees cooler, you will get the kind of
results you're looking for.

Craig

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Craig Standley                             cstand@u.washington.edu
University of Washington     http://weber.u.washington.edu/~cstand
Physics
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On Tue, 23 Jun 1998, Mark Hineline wrote:

> Thanks to Sean and Craig for the rock-in-the-fire idea. However, I am
> writing from the Southwestern part of the country where fires are often a
> big, big, no-no.
>
> If the salt has the same properties as a rock, then you could throw the
> rock on the stove and warm it up, right? Seems like the salt has a
> property that the rock doesn't, which is that it diffuses heat throughout
> its mass rather quickly -- which is why it is used in those space
> heaters. No?
>
> (I've done the rock in the fire -- been doing it for over forty years.)
>
> Mark Hineline
>