| http://www.w3.org/ns/prov#value | - For example, the annular space may comprise a fluid, preferably a gaseous fluid such as air, or other insulation material such as any of the known synthetic insulation materials, silica gel, or thermal insulating materials such as are disclosed in U.S. Pat. Nos. 2,967,152, 3,007,596, and 3,009,600, all of which are incorporated herein by reference.
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