In A Historical Dictionary of Indian Food (Oxford University Press, 1998), author K. T. Achaya says that fresh lime soda was "the supreme quencher of colonial thirst" in India.
2 tbsp. simple syrup (made by first boiling
together equal parts sugar and water,
then letting it cool)
1 1⁄2 tbsp. freshly squeezed lime juice
(regular supermarket limes—a k a persian
limes—are the best ones to use)
3⁄4 cup soda water
1. Fill a glass with ice cubes, then add simple syrup and lime juice. Add soda water. Stir well, then serve garnished with a lime slice, if you like.