Each of my bikes has a little patch kit, containing a few patches,
scraper, and a little container of rubber cement. One bike had a slow
leak so I take out the tube, find the hole, scrape it, and prepare to
apply the rubber cement, but the little container of cement is empty.
I used it once, closed it, and it looks like there is cement in it
but, no there isn't.

I pull the patch kit out from another bike, and the rubber cement is
also empty. Very weird. Both little containers look full, but each has
been used once and there is no rubber cement left.

Apparently rubber cement evaporates after you open the containe even
if you close it tightly. And indeed, here is a comment from Amazon on
using something called Bestine:

http://www.amazon.com/Best-Test-Best.../dp/B000HF6UR0

5.0 out of 5 stars SOLVENT FOR RUBBER CEMENT November 10, 2011
By Victor J. Tennery
Amazon Verified PurchaseIN MY WORK I USE A CONSIDERABLE AMOUNT OF
RUBBER CEMENT - CONSTANT PROBLEM IS EVAPORATION OF THE CARRIER IN THE
LITTLE BOTTLES - RATHER THAN DISCARD, YOU CAN SALVAGE USING THIS
THINNER - CAN NOT BUY THIS ITEM LOCALLY - THIS SHIPMENT ARRIVED
PROMPLY AND WAS IN PERFECT CONDITION.

My neighbor had some rubber cement and I borrowed it and the patch
worked fine, but now I need to get more patch kits. I think they do
have some that don't use rubber cement but I don't know if those are
as good. At least now I'm warned - I've been riding with patch kits
that would be useless if needed.