Why are people who swallow mercuric salts, copper salts or lead salts given egg white or milk?

Answer 1

Never heard that before, but presumably the proteins in egg white or milk complex to the toxic cations and thus take them out of solution. That at least is the principle behind the cyanide 'antidote': An iron salt is ingested, to which the cyanide complexes. EDTA is also ingested, which complexes to the whole, compelling it to precipitate from soln. I think.