Cream the butter and blend in the ginger and the molasses. Blend in the milk. Dissolve the soda in water and stir it into the mixture. Work in the flour 1 cup at a time. When the dough is smooth, pour it onto greased baking sheets in 2 by 4 inch rectangles. Leave space between, as they will spread during baking. Bake in a preheated 400 degree F oven for 10 minutes, or until the gingerbread is cooked all the way through. Remove, cool and serve.