Song lyrics - Heavy Mind

This one is sung to a 6/8 melody with instrumental breaks and a -- bridge? -- also instrumental, between the second and third verse. Musically I'm happy with the song, but I still wonder if the words are done, for a few reasons: in some places they don't scan as smoothly, even though I know how they'd be sung to make them fit. I wonder if they should be sung slowly, rather than with the swaying lilt that I've set them to. Also, I wonder if I either stretch or carry the metaphors too far, and if the word choice gives the right effect.

I wrote it a year and a half ago and have since become used to it, so maybe it needs fresh eyes. Any thoughts or suggestions are appreciated.
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Heavy Mind

Give me a mind as heavy as water
set over a heart as light as a flame.
My heart it will set my thoughts to boiling
and keep them pure - most of the time.

(instrumental break)

Give me a heart that's lighter than air
to hold up a mind as heavy as stone.
May heart lift the mind high over the surface
to contemplate its provenance below.

(break)
(bridge - different melody, slightly slower tempo, etc.)

And if the flame ever should go out
or rarefied gas escape into space,
though stone it may sink, and water may settle
my mind will remain intact, while I wait

for steel to spark, and fan into flame
if helium won't return.
Water and earth in time will bring forth
a fuel that is fit to burn.