The combination of a vacuum.
Thermos vacuum insulated glass double wall flask.
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Tea infuser water bottle with double walled glass vacuum insulated stainless steel loose leaf filter flask thermos portable mug tumbler with strainer 3 8 out of 5 stars 13 19 99 19.
A thermos then goes one step further.
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In many thermoses you can actually unscrew and remove this glass envelope.
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The glass envelope is fragile so it is encased in a plastic or metal case.
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Invented by sir james dewar in 1892 the vacuum flask consists of two flasks placed one within the other and joined at the neck the gap between the two flasks is partially evacuated.
The glass is silvered like a mirror to reduce infrared radiation.
Inside a thermos is glass and around the glass is a vacuum.