Liberate memory of DOM ! Need Help ! Please...

Liberate memory of DOM ! Need Help ! Please...

Hello everybody ! 

Sorry for my English... I'm French...

I've a big question ! I'm working with my iPhone, but my question is jquery and not jquery mobile so :

I've a structure of page : 
<div id='art'>
      <video id="vid"....></video>
</div>

To empty my div, I make $('#art').html('');
But when I test : 
var movie=document.getElementById('vid');

$('#art').html('');
alert(movie); 
I have an object [html Video].... and not null...
So it's in my dom and in my memory...  so :
my method is bad ?
How can i make to liberate my memory ? 

Thanx a lot ! Very !