Raising agents are the means by which air or gas is introduced into flour mixtures to make them rise while cooking.
Water trapped in a flour mixture becomes steam and when it is heated helps the mixture to rise. Yeast produces carbon dioxide and water during fermentation and this expands rapidly during cooking thus causing the flour mixture to rise.
Bicarbonate of soda is an alkali and when it is mixed with an acid such as cream of tartar it gives off carbon dioxide which gives an open texture to cakes. When using a mixture of bicarbonate of soda and cream of tartar it should be used in the proportion of 2 of bicarbonate of soda to 1 of cream of tartar, as cream of tartar has a more powerful action.
Whisked eggs are also used as a raising agent especially in light sponge cakes where no chemical raising agent is used.
Baking powder is a commercially prepared chemical raising agent – the main ingredients being bicarbonate of soda and cream of tartar. For more information see the entries for, , .