Name That Leavening Agent

Yes, its been slow lately, trying to find something to blog about. So here's a pioneer game for you: Name That Leavening Agent. As you know, pioneers used both chemical and microbial agents to leaven their bread. In either case, they trapped gasses in the dough. From the following recipe, see if you can guess what is causing the gasses that leaven the bread. The recipe comes from The Practical Housekeeper, 1857. Similar recipes were referenced in the Deseret News of our era.

To Make Excellent Bread Without Yeast
Scald about two handfuls of Indian meal; into which put a little salt, and as much cold water as will make it rather warmer than new milk; then stir in wheat flour, till it is as thick as a family pudding, and set it down by the fire to rise. In about half an hour it generally grows thin; you may sprinkle a little fresh flour on the top, and mind to turn the pot round, that it may not bake at the side of it. In three or four hours, if you mind the above directions, it will rise and ferment as if you had set it with hop yeast; when it does, make it up in soft dough, flour a pan, put in your bread, set it before the fire, covered up, turn it round to make it equally warm, and in about half an hour it will be light enough to bake. It suits best to bake it in a Dutch oven, as it should be put into the oven as soon as it is light.

Post your answers in the comment box. I'll send some free saleratus to the winner, drawn at random from all correct answers submitted.