This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
This property is due to the presence in egg of a tenacious, gluelike or viscous substance called albumin. Albumin has the power of holding air beaten into it, or gases formed in the mixture containing it, and of stretching as a result of this.
Air-Holding Power of Egg Reduced by Fat - Egg-yolk is very rich in fat. This is the reason that egg-white is better than the yolk for giving lightness and looseness of texture, and accounts for the direction, familiar to every housekeeper, not to permit any of the yolk to escape into the white when separating eggs, if the white is to be beaten stiff. In cakes in which the air-holding quality of egg-white needs to be used to greatest advantage, the egg-white is beaten alone and is folded lightly into the mixture at the last minute, so that the fat in the mixture may not reduce its viscosity.
Air-Holding Power of Egg Increased by Sugar - In limited amounts sugar increases the tenacity or viscous properties of egg. This fact is interestingly illustrated in cakes, where the addition of sugar, within limits, increases the lightness of the cake. When sugar is added to beaten egg-white, in limited amounts, it increases the air-holding property of the egg, and the meringue is lighter than the beaten egg alone. When the sugar is added to unbeaten egg-white, in limited amounts, and the two are beaten together, not only can the product be made very light but a meringue made in this way holds the air for a much longer time than when it is made by beating the egg first.