On Food and Cooking

invention by — appropriately — a French
scientist and balloonist, J. A. C. Charles.
Charles’s law is this: all else equal, the
volume occupied by a given weight of gas is
proportional to its temperature. Heat an
inflated balloon and the air will take up more
space, so the balloon expands. Similarly, put a
soufflé in the oven and its air bubbles heat up
and swell, so the mix expands in the only
direction it can: out the top of the dish.
Charles’s law is part of the story, but not
the whole story — it accounts for about a
quarter of the typical soufflé rise. The rest
comes from the continuous evaporation of
water from the bubble walls into the bubbles.
As portions of the soufflé approach the
boiling point, more liquid water becomes
water vapor and adds to the quantity of gas
molecules in the bubbles, which increases the
pressure on the bubble walls, which causes the
walls to stretch and the bubbles to expand.