When cleaning gold with nitric acid, what is the use of the mercury?

It's a chemistry question. I want to know what use the mercury has in combination with the nitric acid to clean impure particles from gold and returning it to its powder form. And as a bonus pertaining to this question, what is nitric iodine and how is it obtained? Can it be created? Or do I have to somehow find it on a shelf? I'm looking to do an experiment with nitric acid, mercury, and silver on down the road; but I hear that nitric iodine could be of more use for later experiments.