Baking powder and baking soda are essentially the same thing, which may account for frequent confusion between them.Both are kitchen chemicals that give off carbon dioxide gas to provide leavening in a dough or a batter (another confusing differentiation!)

Both baking powder and baking soda are relative newcomers to cooking. Before the turn of the 19th century, all leavening in baking was done with yeast, by spirits -- which expanded into a gaseous state when heated -- or by manually beating in air -- many recipes called for "beating for an hour or two."

The breakthrough was an American discovery, in 1790, that pearl ash, a refined form of wood ash, could make cakes rise. By 1792, 8,000 tons of it were exported to Europe. The use of this leavening was restricted to highly spiced cakes such as gingerbread, however, because of a soapy taste which had to be masked.

Bicarbonate of soda, another name for baking soda, followed pearl ash. It, however, bore the blame for dyspepsia, a common complaint at the time. And unlike our modern refined versions, it had to be dissolved in a little water before being added to dough, as in the original recipe for Toll House Cookies.

By the mid-19th century, baking powder was introduced, a mixture of bicarbonate of soda with a mild acid, either cream of tartar or tartaric acid (obtained by scraping down the insides of wine vats after grape juice has fermented and been removed), plus rice flour, added as a filler and to absorb moisture. This is what we know as "single-acting" baking powder, which is no longer available in commercial form.

When single-acting baking powder was in common use, cakes had to be mixed zip-quick and popped straight into the oven, as the leavening gases were released all at once, as soon as the baking powder came into contact with the moisture in the batter. If the batter was allowed to sit around before baking, the carbon dioxide would dissipate and the cake would never rise when baking. This made baking a much trickier business than it is today. However, some modern bakers contend we have sacrificed a supremely fine grain in our baked products with the switch to "double-acting" baking powder.

The more familiar double-acting baking powder in our modern kitchens still contains bicarbonate of soda as its alkaline ingredient, plus a combination of acids to provide a double action: leavening gases are released once on contact with moisture, and again during baking.

Substances that neutralize acids and combine with them to form salts are known in chemistry as bases; alkalis are bases that are soluble in water. Bicarbonate of soda, or baking soda, is an alkali that does not have the caustic qualities familiar to us in items such as washing soda or lye. In fact, the flavor of baking soda is even desirable in such foods as soda bread, scones and old-fashioned brick-oven bread.

Modern baking powder combines an alkali (baking soda) and an acid: calcium acid phosphate, sodium pyrophosphate or sodium aluminum sulphate (the sodium in these ingredients is a problem for those with high blood pressure and those on low-sodium diets). In this double-acting baking powder, only a small amount of the carbon dioxide is released when it is moistened; most of the gas is released during heating, or when the dough or batter is cooking. Therefore, when single-acting baking powder went out of use, so did tiptoeing past the oven on baking day to prevent the cakes from falling. Thus, modern baking powder is much hardier than what was available to our grandmothers.

There is an alternative for those who would like to hearken back to old-fashioned baking practices, which may have been more tenuous than today's, but gave results far preferable (for perfectionists) to those we get with double-acting baking powder. One who feels this way is Edna Lewis, author of "The Taste of Country Cooking" (Knopf, 1976). "For my tastes," she says, "double-acting baking powder -- the only kind you'll be able to buy now -- contains so many chemicals that it gives a bitter aftertaste to baked goods, and even more so if the product is held over a day or so."

To make your own chemical-free baking powder, combine 2 teaspoons cream of tartar with 1 teaspoon baking soda; add 1 teaspoon cornstarch if you plan on storing your supply. This is good to know if you are caught in the middle of a baking spree with no baking powder in the house. But remember: you've just made single-acting baking powder, so it must be used more carefully than the double-acting kind. Work quickly and bake your mixture just after adding the baking powder, especially with a thin batter, because the carbon dioxide gas spreads through more quickly.

This is also a good solution if you've suddenly discovered that your supply of baking powder has gone inactive. All types of baking powder lose potency on standing, so it's best to buy in small quantities and store tightly covered. If you suspect your tin might be too old, test by mixing 1 teaspoon baking powder with 1/3 cup warm water; if it fizzes, the powder's still good.

To determine the working differences between baking powder and baking soda, we have consulted the most renowned pastry chef in this country, Albert Kumin, formerly of the Four Seasons and Windows on the World restaurants, the highly acclaimed pastry chef in the Carter White House, and now the head pastry instructor at the Culinary Institute of America, in Hyde Park, N.Y.

According to Kumin: Baking powder is used in doughs or batters that require a vertical leavening action. When you would like a baked item to rise up rather than spread out, use baking powder. Baking soda is used when you want a baked product to spread out, rather than rise up.

However, there is an important exception: When you're making something that incorporates an acid among the ingredients (buttermilk, lemon juice, vinegar, molasses) and in which you desire vertical leavening, you might choose to use baking soda rather than baking powder to get a better, more refined taste. The leavening action when the baking soda reacts with the acid in the dough would be the same as with baking powder, but the taste would be better.

Relying on the chocolate in a recipe to provide the acid for a chemical reaction with baking soda, as some chefs recommend, is not advisable, because the acid content of chocolate varies from batch to batch, depending on how the chocolate was processed.

The reason baking soda is used in chocolate chip cookies (to get to a point raised earlier in these pages) is that Americans prefer a flatter, more spready cookie, rather than a cake-like cookie which rises higher. Both baking powder and baking soda would work well in a chocolate chip cookie recipe, depending on what kind of cookie you prefer.