Conceive these images in Air

Conceive these images in Air,
Wrap them in flame, they're mine;
set against granite,
Let the two dull stones be grey,
Or,formed of sand,
Trickle away through thought,
In water or in metal,
Flowing and melting under lime.
Cut them in rock,
So,not to be defaced,
They harden and take sharp again
As signs I've not brought down
To any lighter state
By love-tip or my hand's red heat.