Baking yeast by John Spottiswood.

This is used as a leaven in breads, coffeecakes, and pastries like croissants and brioche. It works by converting sugar into carbon dioxide, which causes the dough to rise so the bread will be light and airy. Yeast comes either as dry granules or moist cakes. It becomes less potent after the expiration date stamped on the package, so dough made with it may take longer to rise, or not rise at all. If the potency of the yeast is in doubt, test or "proof" it by putting some of it in warm water (105° - 115° F) mixed with a bit of sugar. If it doesn't get foamy within ten minutes, you'll need to get fresher yeast.