  Calcium hydride canister.  
This canister is meant to be used to generate a volume of hydrogen gas to inflate a weather balloon.  I think you poke out the indentation on the top and add water, but I'm not sure. 
Source: eBay 
Contributor: Theodore Gray 
Acquired: 28 February, 2009 
Text Updated: 1 March, 2009 
Price: Forgot 
Size: 6" 
Composition: CaH2
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