Is it not true that there are no perfect vacuums available to directly measure c in?

By the way, the 474 mile path-length is not a perfect vacuum.

In this case the medium was a virtually perfect vacuum.

Do not expect to pull a perfect vacuum, so the actual decrease in mass should be less than 1.2g/L.

Actually, for an electrical discharge to occur, some material has to be present, so it could not happen in a 'perfect' vacuum.

No storm could touch us at the axis, riding as we were in almost perfect vacuum.

The space within them is much clearer than in the spiral arms, almost a perfect vacuum.

It wouldn't even need to be a perfect vacuum.

Outer space is the closest natural approximation to a perfect vacuum.

Space itself is not cold; the only loss of heat is by radiation into or through an almost perfect vacuum.