well Q it would seem most appearant that estimate that you have posted in you link/qoute is one that is a general statment of the speed of light.
consider the event not all light is of the same frequencey, or length some light is only 100 angstroms, and some only 4000 angstroms, some 10,000 Angstroms, wave forms get as long and several meters and so on.
consider also that each wave from of light has a emf feild, simply light has electromagent properties.
the shorter the wave form the higher the eletromagentic properties.
if you said that red light traveled at 186,000 miles per second, and it was 8,000 angstroms in length, what would be the speed of light of a x-ray that is only 100 angstroms in length.
plainly 100 is 80 times smaller than 8,000.
looking a closer we see than gamma is even smaller, and has a greater effect on mass than light, like wise a electron, and as said before a x-ray, in order for such energy to have a effect it must have a speed at which energy can react that is not back ground energy radation. or seen another way, the x-ray, gamma, and electron have more energy and therfore react with other energy slowing it down.
these event pretain reagrless if it is in a vaccum or air, as long as radation or energy exist in medium the x-ray will react at a greater rate than other light. simply conservation of energy and engery expenditure demonstrate this.
common sense demonstartes that light with different lengths will have different speeds and different electromagnetic reactions.
example radio wave do not respond as geen light, plainly ther are different properties with different light. like wise different speeds.
in general if a x-ray is 80 times less than red light, which tavels at 186,000 mi per sec then
186,000 divided by 80 = 2,325
the speed of a x-ray is then 183,675 mph or minus 1/80th of the speed of light red.
beta patricles are as slow as 150,000 miles per second.,
the event of emf is the coupling speed of light , in organized dierction which is 166,000 miles per second. therfore one can see than random light is different than organised coupling of light and for that reason has two different speeds, as well they have two different effects.
DWAYNE D.L.RABON