I use them to store dipped autoradiographs (emulsion must stay dry 
during exposure), and frozen sections [fixed or unfixed] on slides 
saved in the freezer or refrigerator for immuno or in situ 
hybridization.  The plastic slide boxes are sealed for air-tightness 
with 'stretchy' black electrician's tape, and for light-proofing with 
aluminum foil if needed.  In addition to keeping tissue very dry and 
thus preventing any unwanted chemical reactivity, the dessicant 
prevents condensation of moisture from the atmosphere on slides as 
they warm to room temperature in preparation for further processing.

Note: when storing frozen tissue blocks for future sectioning, the 
opposite problem is encountered -- the goal is to prevent dessication 
of the block and resultant gummy or rubbery tissue.  In that case I 
seal the block airtight with some water ice included, so that 
sublimating water vapor is in balance with the air surrounding the 
tissue.

-D

>I have never heard of anyone using these things to store slides.  What is
>the purpose?  Is this something recommended for long-term storage?  I
>would be interested in a vendor if you(s) think this is a good thing to
>use.
>
>Thanks,
>
>Trisha