A worker at the site of the old St. Joseph’s Hospital fell through a grate into a 25-foot-deep hole Tuesday morning, but did not appear to suffer serious injuries, Lowell fire officials said. The man was conscious and alert after the fall. He was taken to Lowell General Hospital to be checked out, said Deputy Fire Chief Michael Cushing. The former hospital site is being prepared for demolition. The University of Massachusetts Lowell has bought the property, Cushing said. It was as if “someone pulled the floor right out from under him,” said Cushing.