Well that certainly is a weird one. I'm sure it's not an insulator. It reminds me of fire grenades and target balls.
Could it be a "gang" fire grenade? If it's sealed and the fluid is original, then my guess is fire grenade. The fluid might be carbon tetrachloride in that case, so be warned. Original fire grenade are usually marked, I think, and this has no markings?
Other guess: if fluid is alcohol, might be an alcohol-fountain demo (they make hand-size ones now: hold the bulb, body heats boils the alcohol, which jets out in a little fountain and cools off [repeat]; all happening in a sealed glass body). But this is meant to sit horizontally, not vertically, so perhaps not.
It certainly is strange, and I love a good whatsit. Don't know what it is, and I am still thinking about it...