Ammonia powder is ammonium bicarbonate, a leavening agent that is a precursor of today's baking powder and baking soda; it must be ground to a powder before using. It is also known as hartshorn, carbonate of ammonia or powdered baking ammonia. It is still called for in some recipes especially for cookies.
I would suggest substituting an equal amount of double acting baking powder for the powdered ammonia (hartshorn) in a recipe. (Your kitchen would stink of ammonia while the cookies baked, but the cookies would not taste or smell of ammonia.)