To whom does macbeth refer when he says, "what seem'd corporal melted as breath into the wind

You have to look at the lines preceding the line in order to know who he is speaking of. 

Whoever it is apparently dissolved into thin air. Then you can see there is a stage direction that states "Witches vanish." This is who Macbeth is referring to when he makes that statement. 

(1) native americans used the area as their home for about 11000 years: : : (b) transportation routes(2)