请问如何动态添加<span>呀

<html>
<meta http-equiv='content-type' content-type='text/html' charset='gbk'>
<body>
<script language='javascript'>
var spanCount=0;
function _addNew()
{
  var newSpan=document.createElement("span");
  newSpan.id="_span"+spanCount;
  spanCount++;
  newSpan.innerHTML=newSpan.id;
  document.all.frm.appendChild(newSpan);
}
function _check()
{
  for (var i=0; i<spanCount; i++)
    alert(eval("document.all._span"+i+".innerHTML"));
}
</script>
<form id='frm'>
<input type='button' value='add' onclick='_addNew();'>
<input type='button' value='check' onclick='_check();'>
</form>
</body>
</html>