The sky has always been blue, but it took a scientist from Leighlinbridge,Co Carlow to discover the reasons and explain why, writes Mary Mulvihill.
It's amazing where an obsession with dust can get you. Take John Tyndall, for instance, whose near-fanatical interest in what he called the "floating matter of the air" led him to discover why the sky is blue, and to prove that Pasteur's germ theory was correct.
It wasn't bad going for a boy from the small Co Carlow town of Leighlinbridge whose early education went no further than the local national school. But then, John Tyndall was no ordinary boy, and he did eventually study for a PhD at Marburg University in Germany.
Tyndall became one of the great scientists of the 19th century, a towering figure in Victorian science. He was a friend to Charles Darwin and Michael Faraday, succeeding Faraday as director of the prestigious Royal Institution in London (still home to the popular Christmas science lectures for children). And when Tyndall went on a lecture tour of North America, this talented populariser took the US by storm.
But what of his interest in dust? By studying and experimenting with particles in the air, he came to discover what is now called the Tyndall effect: that the sky is blue because particles in the atmosphere scatter the short-wavelength blue light.
This also explains why the sun looks red. If there was no atmosphere and the full spectrum of sunlight could reach us, the sun would look white. But the atmosphere scatters the blue wavelengths, leaving the red to filter through.
Tyndall also demonstrated that our lungs filter dust from the air, and significantly, that bacteria are present in air everywhere and that food kept in sterile air does not putrefy. These findings provided the first proof of Pasteur's germ theory of disease and delivered the final blow to the idea that life could spontaneously generate from the air.
Tyndall next invented a sterilisation process (tyndallisation), which is still used in the food industry. Food is heated briefly but repeatedly so that organisms not killed the first time will be killed the second time (in pasteurisation, the food is heated once but for longer).
All of this is only a small part of the man's contribution to science. He was also the first to explain how glaciers move (like ice skaters, on a layer of meltwater), and he discovered the principle now used to send signals down fibre optic cables (called "total internal reflection", Tyndall demonstrated it in a pipe of water). He also first proposed that changes in atmospheric gases could cause a greenhouse effect, in honour of which the University of East Anglia named its Tyndall Centre for Climate Change.
Pugnacious by nature, Tyndall also provided one of the historic flashpoints in the science versus religion debate that followed the publication of Darwin's controversial book, On the Origin of Species. He even found time to become a keen mountaineer and was the first person to climb the Weisshorn.
But his later years were plagued by ill health and insomnia. Tragically, 110 years ago today his devoted wife gave him a large dose of sleeping draught, thinking it was indigestion remedy. Despite emetics and stomach pumps, the great Irish-born scientist died later that evening.