Mix the soda and the salt with the flour, and sift them several times so they will be thoroughly mixed. Rub in the butter evenly. Stir in lightly with a fork enough sour milk to make a dough just stiff enough to roll. The dough can be left very soft if the board is well floured and the rolling-pin is used very lightly, patting the dough rather than rolling it. Roll it out quickly an inch thick. Cut it into small rounds. Bake in a quick oven twenty to thirty minutes. The dough can be rolled half an inch thick, and two rounds placed together with a small bit of butter between. They are then called twin biscuits. These biscuits may be made of sweet milk, in which case two rounding teaspoonfuls of cream of tartar must be used with the soda and mixed with the flour.