Combine 3 and 1/2 cups of finely chopped Rhubarb with 3 and 1/2 cups of sugar. Add 1/3 of a cup of vinegar, 1/2 a cup of lemon juice and 20 cups of cold water. Place mixture in a sealed container and leave for 2-3 days. Strain to remove rhubarb. For a sparkling cordial, bottle then leave for one week before drinking or for a flat cordial, boil then bottle the hot liquid.