Tonic syrups, which get their brown hue from cinchona bark, are meant to be mixed with the spirit and soda water to create a gin and tonic. These include, from left, citrusy Small Hand Foods, deliciously bitter Wilks & Wilson, and woodsy TomR's. Bottled tonic waters are infused with purified quinine. Choose those sold in glass, which retains carbonation. Q Tonic is sweetened with agave, while Fever Tree uses cane sugar. Zesty Fentiman's is infused with kaffir lime leaf and lemongrass.