This cannot be done accurately without a thermometer. The milk bottle should be placed upon a rack in a kettle of water, with a clean thermometer inserted through the cover of the bottle. Heat the water slowly, and watch the thermometer. When it reaches 155° F., see that the water becomes no hotter. Set the kettle on a rack on the stove top, or use a simmering burner with rack and asbestos mat. It is difficult to keep the temperature even, but it should remain at 140-155° F. half an hour. At the end of half an hour, the bottle should be removed, and cooled as rapidly as possible in running cold water.