Rinse the clams in clam liquor. Remove the black caps. Strain, and reserve ½ cup clam liquor. Chop the clams and set aside. Fry the salt pork until brown and crisp. Drain on paper. Saute the onion slices, then add the potatoes. Sprinkle with salt and pepper. Saute for 10 minutes. Add the chopped clams and ½ cup clam liquor. Cover with water and cook for 20 minutes. Add the fried pork and fat to the clams and vegetables. Then heat the milk and add to the chowder. Add butter, season to taste, and serve.