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This episode tells the story of how the Emperor Constantine brought Christianity to the western world. In AD 312, Rome was in crisis. The empire had been divided into four parts, each with its own emperor who fought one another. Constantine intervened and united Rome, using military might and a new religion - Christianity.

Julian Richards recalls how, after years of raiding, England's resistance was so weakened that, in the early 11th century, the Vikings were finally able to seize the throne. In other parts of the British Isles however, they gained and maintained power by integration.

In part two, Professor Al-Khalili looks at the 19th century chemists who struggled to impose an order on the apparently random world of the elements. From working out how many there were to discovering their unique relationships with each other, the early scientists' bid to decode the hidden order of the elements was driven by false starts and bitter disputes. But ultimately the quest would lead to one of chemistry's most beautiful intellectual creations - the periodic table.

Francisco de Orellana failed to find El Dorado, but discovered the Amazon. Early in 1541, a rumor swept Quito that beyond the mountains, there lay a land richer than Mexico, or even Peru - a land of gold. The ruler of this land was so rich that he covered himself with gold dust every day and washed it off every evening. He was "the golden man", El Dorado. They had tree goals: to find La Canela, the land of cinnamon: to assess new lands for colonization; and to find El Dorado. On march 1541, They marched eastwards with more that 200 Spanish troops and thousands of native servants.

The Greek tale of Jason and the Golden Fleece has been told for 3,000 years. It's a classic hero's quest tale - a sort of ancient Greek mission impossible - in which the hero embarks on a sea voyage into an unknown land, with a great task to achieve. He is in search of a magical ram's fleece, which he has to find in order to reclaim his father's kingdom of Iolkos from the usurper King Pelias. Michael Wood discovers a story of heroism, treachery, love and tragedy that would make Hollywood proud.

There are 200 million insects for each of us. They are the most successful animal group ever. Their key is an armoured covering that takes on almost any shape. Darwin's stag beetle fights in the tree tops with huge curved jaws. The camera flies with millions of monarch butterflies which migrate 2000 miles, navigating by the sun. Super slow motion shows a bombardier beetle firing boiling liquid at enemies through a rotating nozzle. A honey bee army stings a raiding bear into submission. Grass cutter ants march like a Roman army, harvesting grass they cannot actually eat. They cultivate a fungus that breaks the grass down for them. Their giant colony is the closest thing in nature to the complexity of a human city.