Actually I picked that one precisely because whilst you can't see it, any halfway decent chemistry student could prove the existence of nitrogen in the atmosphere beyond any shadow of a doubt. They MIGHT be lying in a vast conspiracy, but it would be absurd to suggest so in the face of multiple, credible scientific proofs, to the extent you'd make yourself look like a loon by denying it. Humans have an enormous capacity for that sort of thing, but it does them no credit when they use it.