Sentence:  A waiter brought his aperitif, which was a small scotch and <WSD>soda</WSD>, and as he sipped it gratefully he sighed./     ‘Civilized,' he said to Mr. Campion. ‘Humanizing.' ... ‘Cigars and summer days and women in big hats with swansdown face-powder, that's what it reminds me of.' 
Meaning of the word marked with <WSD> tags:  Carbonated water (water impregnated with pressurised carbon dioxide, originally made with sodium bicarbonate).