error when using jquery dialog?

every time i try using jquery dialog i get error null when debugging using firebug, here is the code

<script type="text/javascript" src="js/jquery-1.4.2.js"></script>

    <link rel="stylesheet" type="text/css" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.7.1/themes/base/jquery-ui.css">
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.7.1/jquery-ui.min.js"></script>
      <script>
      $(document).ready(function() {
        $("#dialog").dialog();
      });
      </script>

    <div id="dialog" title="Dialog Title">I'm in a dialog&开发者_如何学运维lt;/div>

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It could be that your jQuery.js file doesn't exist in that path. Replace it with the Google hosted one.

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>

Also you should add <script type="text/javascript"></script> where your jQuery code is.

<script type="text/javascript" src="js/jquery-1.4.2.js"></script>

<link rel="stylesheet" type="text/css" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.7.1/themes/base/jquery-ui.css">
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.7.1/jquery-ui.min.js"></script>
  <script type="text/javascript">
  $(document).ready(function() {
    $("#dialog").dialog();
  });
  </script>

<div id="dialog" title="Dialog Title">I'm in a dialog</div>

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Your <link> element isn't closed. I'm guessing that it's somehow picking up and squashing the dialog div, as anything in a link tag I would presume is ignored and not placed on the DOM.