Remove thorns and blossom ends from fully ripe cactus. Cut into small pieces and crush. Add 1 cup water. Put in pan and bring to a boil and simmer 10 minutes. Place in jelly cloth or bag and squeeze out juice. Measure 3 1/2 cups into a very large pan. Add lemon juice (about 2 lemons) to cactus juice. Add sugar and mix well. Place over high heat and bring to a boil, stirring constantly.
Remove from heat, skim off foam and pour quickly into glasses. Seal at once or cover with 1/8 inch paraffin. Let set 5 days before serving. Makes 9 glasses.