The dissolving Morgul blade

[Leaf]

I have a small question regarding this:

Originally Posted by The Lord of the Rings, The Fellowship of the Ring, Flight to the Ford

He stooped again and lifted up a long thin knife. There was a cold gleam in it. As Strider raised it they saw that near the end its edge was notched and the point was broken off. But even as he held it up in the growing light, they gazed in astonishment, for the blade seemed to melt, and vanished like a smoke in the air, leaving only the hilt in Strider’s hand.

What was the cause for the disintegration of the Morgul blade? Did it disintegrate because it was being touched by Aragorn (like the movie suggests), or rather because it was exposed to the growing light?!

I am not a native speaker but it seems to me that the word even suggests a strong (time-wise) correlation between the dissolving of the Morgul blade and the growing light. I think it's notable that Aragorn (contrary to PJ's film) can touch and lift the blade for seemingly some time before it vanishes.

What do you think?