de\serialize xml contains lists of object to lists of objects

I'm trying to save lists of object to a xml file. But when I'm save once it delete the before saving. I mean if I save on xml file 1 list of "Radar" and than save on the save xml file list of "Observer" it saves only the last list I save. There is any way to save them both ? My code to save :

XmlSerializer serializer = new XmlSerializer(typeof(List<Observer>));

                using (TextWriter writer = new StreamWriter(@"C:\Users\user\Desktop\MapSample\bin\Debug\ListObserver.xml"))
                {
                    serializer.Serialize(writer, list);
                }

Hope you understand. Thanks in advice.