from "The complete guide to the art of modern cookery", by Escoffier

Lemonade

Dissolve 250 g loaf sugar in 1 litre filtered water. Add the rind and juice of 2 lemons and allow to infuse in a cool place for 3 hours.
Pass through a fine strainer, add asyphon of Seltzer water and serve in glasses with a thin slice of lemon in each.