Add the soda to the potatoes and mix well. Then mix the remaining ingredients in the order given, and combine with potatoes.
Put in a buttered baking dish and steam 2 hours in a moderate oven (preheated to 350°), placing the baking dish in a pan of water to prevent scorching on the bottom.
Remove from oven. Let cool 5 minutes. Remove pudding from baking dish by inverting over serving platter.
Serves 6 to 8.