This is my HTML

<select name=countries id=countries MULTIPLE size=8>

   <option value=UK>UK</option>

   <option value=US>US</option>

   <option value=Canada>Canada</option>

   <option value=France>France</option>

   <option value=India>India</option>

   <option value=China>China</option>

</select>

<br />

<input type=button id=select_all name=select_all value=Select All>

When user click on 'Select All' button, I want all the options in the select box to be selected

$('#select_all').click( function() {

    // ?

});