  Argon signal lamps. 
I'm guessing that these antique bulbs are filled with nearly pure argon, but I don't really know.  Connected to a hundred or so volts they produce a characteristic purple-blue glow over the surface of the electrodes.  Not enough to create usable light, but perfect as a signal light, the kind of thing that today would be done with an LED or neon indicator light. 
Source: eBay seller ramblingguy 
Contributor: Theodore Gray 
Acquired: 28 February, 2009 
Text Updated: 12 March, 2009 
Price: $10 
Size: 3" 
Purity: 90%
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