Dewar put a vacuum between two walls to block heat

In 1892 Scottish scientist James Dewar was doing experiments on the specific heat of the element palladium, which needed to be held at a very cold, stable temperature. His problem was that anything that cold lost its chill quickly, because the air around it carried heat into it by conduction and convection.

His solution was unusual: he made a brass chamber and enclosed it in a second chamber, then evacuated the air between the two. With almost all the gas pumped out, there was no longer any air for heat to conduct or convect across the gap. The near-vacuum between the walls became the insulation, and the contents stayed near their set temperature.

The principle is the key physical insight. Insulation normally works by making heat's path harder — a thick jacket, a slow gas, a porous foam. Dewar's flask works by removing the path altogether. With the air gone, heat physically has almost nothing to cross the gap, so only the small amount that gets through the walls and the neck matters. The double-walled, evacuated vessel was the result of his cryogenics research and is why it is called a Dewar flask.

Dewar never patented the design, and it was quickly commercialised. In 1904 two German glassblowers, Reinhold Burger and Albert Aschenbrenner, built a more robust everyday version, named it Thermos, and claimed both the product and the trademark — later winning a court case against Dewar himself for the name.