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Hello! This week Dr. Who gets to mention, as we unlock the story of a slimy element, that makes people stink of garlic and turns their fingers black. With the tale of tellurium, here's Peter Wothers.

Tellurium, it sounds like a Dr. Who monster and in a number of ways this element does have a few properties that would make it suitable for any good outer space, sci-fi horror movie. For a start, like many space monsters, it comes from slime; to be precise it is extracted from anode slime, a waste product formed during the electrolytic refining of copper. Its special power, well in the form of cadmium telluride, it can capture solar energy. Far from being used for evil though, this compound has been used in some of the most efficient solar cells for the generation of electrical power.

Every good monster must have a secret weapon and tellurium is no exception. It gives its enemies garlic breath, really bad garlic breath. A dose of half a microgram, hardly even visible would give you garlic breath for 30 hours, Oh! And it also gives its victim black patches on the webbing in between the fingers, but few people would get close enough to notice this. Like a certain well-known vampire, tellurium was first discovered in Transylvania. This was in 1783 by Franz Joseph Muller von Reichenstein, the chief inspector of the mines there. He was having particular problems with the analysis of an unusual gold containing ore. Eventually, he managed to isolate a new metal from the ore and he called it aurum problematicum. He sent a sample to the German chemist Martin Klaproth, who confirmed it was a new element and gave it the name tellurium. But to properly understand why he called it this, we need to go way back in time and look into space.

When early man looked up at the stars at night, he noticed certain heavenly bodies that moved through the fixed pattern of the stars. These were the planets Mercury, Venus, Mars, Jupiter and Saturn. Two other great bodies also seemed to circle the earth, namely the Sun and the Moon. Altogether then there were seven such heavenly bodies and seven was a magical number. Early man also knew of just seven metals, gold, silver, copper, iron, tin, lead and mercury; surely this could be no coincidence. In the same ways that rays from the sun nourish plants and are essential for their growth, it was thought that the invisible rays from the planets helped nourish metallic ores in the ground. Each planet was thought to have a particular influence on one metal or its ores. Chaucer described this connection in the 14^th Century. The Sun is associated with gold, the Moon with silver, Mars with iron, Saturn with lead, Jupiter with tin and Venus with copper and even today, we still keep the same name for both the planet and the element, Mercury. The association between gold and the Sun seems fairly obvious from their colours, similarly the connection between silver and the Moon. The other connections are little more vague. A 17^th Century text quotes, "Iron is called by the name of Mars whether employed for the making of weapons of war, of which Mars was said to be the God or because of the influences from which iron receives from this planet." It is interesting that we now know that the colour of this red planet is due to the oxides of iron. The chemists called copper, Venus both by reason of the influences, which possibly it receives from that planet and of the virtue it had in diseases seated in the purpose of generation. This is referring to early treatments of venereal diseases, the diseases of Venus. Being the planet closest to the Sun, Mercury moves through space faster than any other. It takes Mercury just 88 days to orbit the Sun, compared to our 365 days. Perhaps, this speedy motion was one of the reasons for the lasting association between the metal and the planet or perhaps it is as described in one book "due to the fact that the element has an aptness to change its figure, a property attributed by the heathens to mercury, one of their false Gods." The connection between the elements tin and lead with Jupiter and Saturn were even more dubious.

Unfortunately, the magic number of 7 metals didn't last. For a while, early chemists, just conveniently passed over antimony, arsenic, bismuth, zinc and cobalt. After all they weren't real metals, but with the discovery of platinum, they could ignore it no more. For a while, platinum was even known as the eighth metal. Still more metals were discovered, but then in 1781, a new planet was discovered, Uranus. Just as the ancient God, Saturn or Cronus was the father of Jupiter or Zeus, the new planet should be named after the father of Saturn, hence Uranus, after the Greek God of the sky. In recognition of this discovery in 1789, Klaproth named a new metal he had discovered after this element, uranium. So in 1798, when Klaproth had the chance to name another element, he named it after the only then known planet in the Solar System that did not have an element named after it, the Earth. In ancient mythology, Tellus or Terra or Gaea was the goddess of the Earth and the wife of Uranus, the God of the Skies. Thus was born tellurium.

A chemist, who takes his inspiration from the heavens, that was Peter Wothers from Cambridge University, telling the story of tellurium. Next time, the illuminating tale of a gas that everyone thought wasn't worth the time of day.

And initially its lack of reactivity meant there were no obvious uses for Neon. It took a bit of imagination from the French engineer, chemist and inventor, Georges Claude, who early in the 20^th Century first applied an electric discharge to a sealed tube of neon gas. The red glow it produced, gave Claude the idea of manufacturing a source of light in an entirely new way. He made glass tubes which could be used just like light bulbs. Claude displayed the first neon lamp to the public on December 11, 1910 at an exhibition in Paris. His striking display turned heads but unfortunately sold no Neon tubes. People simply didn't want to illuminate their homes with red light.

But they did want to see their names written in lights and that's exactly what Georges Claude did next as Victoria Gill will be telling us next time. I hope you can join us. I'm Chris Smith, thank you for listening. And Goodbye!

Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld.org/elements.