Last month, I talked about how bread and other baked items rise, and the different ways and means by which that can happen.  To review: yeast is a biological leavening agent that causes gluten to develop and then rise when mixed with fluid and heat.  Chemical leavening agents, such as baking soda and baking powder, react with an acid and heat to cause breads to rise.  Lastly, mechanical leavening agents, such as whipped egg whites, can also cause breads to rise as well.   Additionally, I pointed out the difference between baking soda and baking powder.  Bottom line: they ARE different, and cannot be substituted for each other.