Perhaps the most useful substance known to modern chemistry, and given off by all other substances when they burn. Discovered by Boris Becher in 1667, it was initially unpopular due to the fact that he gave it the stupid Latin name tinea pedis, which nobody understood (Roman civilisation having long been in decline), & that if you say it while you have a cold, most people will either laugh or feel insulted. The substance enjoyed a revival only when subsequently renamed to its present designation by Becher's student, Georg Stoohl.
Rejected by 18th century cranks (such as Lavoisier), who mistakenly identified it as 'the absence of oxygen', phlogiston quickly fell from favour; from this time only maverick chemist and philosopher de Selby pursued the theory with any seriousness, until it's rediscovery in the late 20th century. For political reasons, de Selby's work was suppressed by (depending on the conspiracy theory du jour) the Pope, Opus Dei, The Elders of Zion, Dan Brown and possibly The Wimbledon and Merton Residents Association.
The most useful property of phlogiston is its negative mass (first identified by Lavoisier, but which he mistakenly attributed to a lack of oxygen). It is this that has secured it a key role in Nasa's prototype Antigravity Drive programme. Obviously the biggest challenge in phlogiston technology has been the issue of storage. Penhaligan and Mulholland recently calculated the critical volume necessary to seriously deviate the Earth's orbit to be 10 litres (9.27 gallons), or roughly approximately roundabout the amount of water in an average paddling pool. There are currently some 3,279 patents pending on phlogiston technology. These range from small-scale applications such as AG-assisted umbrellas and support brassieres, to a plan by environmental artist Christo to elevate an entire section of the Rocky Mountains some 3,000 feet into the air.
Recent Archaelogical Findings
The recent dig at Quidditch-Al-Salami has revealed circumstantial evidence that phlogiston technology may have been known about and employed since ancient times. The ancient Quidditch airport, with its wooden launch structures, hemp tethers and fossilised ballast-camels, seems to have been an entirely human conceit, despite the dubious alien-tech claims of UFO-hugging eccentric Erik von Daniken.