The chemical compounds, for instance, are phosphorous this or that.

(If you speak a romance language, it jumps out at you because the adjective has an extra syllable - e.g. 'fosforo' vs. 'fosforoso'.)

That's true of phosphorous acid (where the name distinguishes it from phosphoric acid), but otherwise English chemical names use the noun form of the positive ion (e.g. phosphorus pentoxide).

> You mean the difference between the element and the quality of glowing?

Or just something containing/made partly of phosphorus (compounds of phosphorus being the most obvious examples, as @ars says): that's common (though glowing with a phosphorous light does have a lovely cadence)

Yes, I think the adjective in it's more modern form usually occurs as 'phosphorescent'.

But it could be phosphorus black phosphorus ?

I don't know anyone who uses that stuff, except maybe Hotblack Desiato.