Mix and sift the dry ingredients. Dissolve the soda in a small quantity of water and stir into the molasses. Combine with the sour milk, then mix into the dry ingredients. Flour the raisins and add to the batter. Mix thoroughly and pour into two greased molds filling two-thirds to the top. (Old baking powder cans used to be the standard molds.) The cover should be tight-fitting, and should be buttered before being placed on the mold. It should then be tied down with a string so that the bread will not force off the cover as it rises. Place the molds on a rack in a kettle containing boiling water which comes halfway up around the molds. Cover and steam for 3 hours, adding more boiling water if needed. Use a string to slice brown bread.