Quinine
The bitter tonic water we know today has its roots in the use of quinine (the bark of the Peruvian cinchona tree is dried and powdered) as a prophylactic against malaria, a property that had been known to the Quechua people (who are indigenous to Peru, Bolivia, and Ecuador) for centuries.
The Jesuits first brought cinchona to Europe in the 1570s, and its use to treat malaria was recorded in Rome in 1631. In fact, it is sometimes known as Jesuit’s Bark. When King Charles II was cured of malaria with quinine at the end of the C17th, it became popular in London. Quinine remained the antimalarial drug of choice until the mid-C20th.
Early tonics consumed for ‘medicinal purposes’ had large amounts of quinine – the effective dose is over 600mg orally every 8 hours for seven days. Modern tonics have a fraction of that quantity and have subsequently no medicinal value whatsoever.
Carbonation
In 1767 Joseph Priestley, an English scientist, built on earlier work by Henry Cavendish and developed a process for carbonating water – that is, for bubbling carbon dioxide through water – and found his ‘soda water’ had a pleasant taste and offered it to friends as a cool and refreshing drink. Priestley referred to his invention of soda water as being his "happiest" discovery.
A Swiss-born watchmaker, Johann Jacob Schweppe, developed a commercial process to manufacture carbonated mineral water and founded the Schweppes company in Geneva in 1783, but the company failed in 1795. However, Erasmus Darwin (the grandfather of Charles Darwin) promoted the product, which slowly grew in popularity. In 1831, King William IV adopted the beverage which could then use the ‘by appointment’ label, and subsequently became extremely popular.
The First Gin and Tonic
The somewhat chequered history of Gin is well documented, but for our purposes we can assume that with the invention of the column still in 1826, in use by the then well-established Gordon’s distillery in Southwark (later it moved to Camberwell) meant that the quality of gin shipped out to the Indian colonies was of reasonably high quality.
So, the scene is set for the invention (or discovery, perhaps) of the simple, classic, timeless cocktail, the Gin and Tonic.
Quinine is unpalatably bitter, and it was customary – certainly amongst British officials stationed in early C19th India and other tropical posts – to mix the powder with soda water and sugar. It isn’t recorded when gin was first added to the mix, but it seems very probable that it originated in British Colonial India at the start of the nineteenth century.
Picture perhaps the Officers’ Mess of the 6th Madras Light Cavalry, or the Ootacamund Hunting Club in the hills outside Madras. Picture Lieutenant Colonel George Bingham Arbuthnot, recently returned from an unsatisfactory hunt, late for mess dinner and very, very thirsty. Also requiring to consume his daily ration of quinine, he impetuously takes a large gin from the Sepoy steward and throws it into his medicine glass, and drains the concoction in a single draught. He savours the aftertaste of the mixture in his mouth, and vows to try the same mixture again when at greater leisure. Perhaps while devising the rules of his new pastime, ‘snooker’ (a variant of the popular billiards game), he persuades others to try the mixture. And the rest, as they say, is history.
Gin
Gin must, by definition, have juniper as it’s predominant flavour. Its current popularity is partly based on the variety and richness of all the other flavours that can be introduced by the addition of further botanicals into the distilling process.
Tonic waters are usually quinine-based, but increasingly there are other mouth-watering flavours available, to match the variety of gins.
But the gin and tonic combination isn’t simply a combination of the flavours of the primary ingredients. At a molecular level, a third flavour is created. There are many people who like neither gin nor tonic, but love the gin-and-tonic cocktail. This is because, when gin and tonic are mixed, quinine and the flavour molecules from the juniper berries combine to make a perceived flavour that is different than just the sum of the individual parts. The molecules from the gin and the tonic can do this because they look alike: the molecules are chemically similar.
I’m indebted to @sciencegeist for the chemistry behind this.