Cut the white parts of two heads of celery into pieces an inch long. Place in a stewpan with half a pint of white stock, an ounce of butter, 2 oz. lean ham, and a lump of sugar. Boil for 15 minutes. Add two pints more stock, a blade of mace, and a little salt. Stew gently for an hour. Press the soup through a coarse sieve. Make it hot again. Pour into a tureen. Add half a pint of boiling milk and serve.