The Contessa de Cinchon was unwell. The wife of the Viceroy of Peru was suffering from debilitating fever, but to everyone's surprise, when dosed with a substance that we now call quinine, she recovered quickly. Very sensibly, the Contessa brought a supply back home to Europe, when she returned in 1638. Much later, when the British were shouldering their white man's burden out in India, they found, like the Contessa, that quinine was by far the most effective treatment for malaria. It therefore became the custom to take daily preventive doses in the form of quinine sulphate, the main ingredient of tonic water. And since the taste was not particularly pleasant, it became the norm to wash it down with gin. And thus, apparently, arose the happy marriage of the quinine tonic and the glass of gin, a recipe which has survived as G & T.
In recent years, however, meteorologists have discovered that gin and tonic provides a very effective means of monitoring the ozone layer. In 1845 the English astronomer Sir John Herschel noticed that a glass of tonic water has a slightly bluish tinge. At first Herschel and his contemporaries assumed that this blueness had something to do with scattering by tiny particles suspended in the liquid, that it acquired its colour in much the way as the sky appears as blue.
But it was noticed later that if you move a glass containing diluted quinine sulphate through the various sectors of the natural spectrum, the solution remains transparent everywhere except at the violet end: here an azure tinge is seen, and if the solution is moved even further into a beam of ultraviolet light, it becomes a vivid duck-egg blue. It became obvious that quinine sulphate is fluorescent.
Fluorescence is a process whereby the absorption of radiation by a substance in one wavelength - usually the ultraviolet - causes that substance to emit radiation in a different wavelength, usually the visible. And the ghostly blue light in gin and tonic is a case in point. The blue emission by the tonic is triggered by the absorption in the solution of the tiny amount of ultraviolet light that remains after the sunlight has been filtered by the ozone layer. So by now it will be obvious why meteorologists drink gin and tonic in the line of duty: if, over an extended period, successive G & Ts can be seen to acquire a steadily deeper shade of blue, it indicates more ultraviolet light and hence a steady weakening of the ozone layer.