function return_comments(id) {
    var url = 'includes/functions/ajax_handler.php?action=return_comments&id=' + id;

    new Ajax.Request(url, {
        evalJSON: true,
        onSuccess: function(response) {
            var comments = response.responseJSON;
            if(comments instanceof Array) {
                // render comments
                comments.each(function(comment) {
                    render_comment(comment[2],comment[3],comment[4]);
                });
            } else {
                // there was an error
                alert(response.responseText);
            }
        }
    });
}