Put lemon juice, syrup, 625 ml water and jam sugar into a wide pot, bring to the boil while stirring occasionally and let it boil for about 4 minutes until it becomes bubbly. Add whiskey, stir in and pour jelly into approx. 4 sterilized glasses (350 ml each). Close the glasses, put them on the lid for about 5 minutes, turn the glasses upside down again and let them cool down. Jelly keeps cool and dark for at least 1 year.