He argues that thermite is the explanation for the presence of iron and aluminium in the spheres.

BBC: Q&A: The collapse of Tower 7

Thermite is a substance that can literally melt steel and is made up of iron oxide and aluminium.

BBC: Q&A: The collapse of Tower 7

However, rather than coming from thermite, the metallurgist Professor Sisson thinks the sulphur came from masses of gypsum wallboard that was pulverised and burnt in the fires.

BBC: Q&A: The collapse of Tower 7

More extensive sampling of the dust has not found any evidence of thermite or explosives, says a report from the US Geological Survey and another from RJ Lee.

BBC: 9/11 conspiracy theories

The former professor of physics, Steven Jones, believes the spheres he has found in the dust from the World Trade Centre site match the spheres you get in a thermite reaction.

BBC: Q&A: The collapse of Tower 7