Mix the sugar with 50 ml (approximately 2 ounces) of water, and bring to a boil. Cook until syrupy in consistency, and remove from the heat. Let cool to room temperature. Rinse the lemons, and slice 1 of them. Squeeze the juice from the remaining lemon. Add the lemon slices and juice to the a bowl with the sugar syrup. Let infuse for 30 minutes.
Rinse and dry the basil. Add the basil and the contents of the bowl to a carafe, then fill with the mineral water. Chill in the refrigerator for 10 minutes before serving. Place the ice in glasses, fill with the lemonade, and serve.