Core and peel 24 medium size green tomatoes; cut into thin slices.
Wash, peel and cut 4 oranges, slice thin: cut up peel
Combine all and add 3 1/2 lbs sugar.
Mix well and let stand overnight.
Bring to gentle boil in heavy kettle and stir often.
Simmer until thickened, about 2 hours.
To test for doneness, drip some of the liquid onto a cold plate. If it sets, the marmalade is done.
Meanwhile sterilize the jars, either by boiling them in a large pot of water for 10 minutes or washing them well and placing them in the oven at 375 F (180 C) to dry for 20 mins. Fill the jars with the marmalade while both are still very hot. Seal with sterile lids and rings. Turn jars upside down for fifteen minutes. Then turn upright. When jars have "pinged" they are done. If a jar lid does not sink down wait 24 hours to see if it goes down. If not, place in the refrigerator.