There isn't one single person who "discovered" gas expansion. It's a fundamental property of gases that has been understood for centuries, with contributions from many scientists.

Here's a breakdown of some key figures and their contributions:

* Robert Boyle (1662): Boyle's Law states that the volume of a gas is inversely proportional to its pressure, at constant temperature. This was a crucial step in understanding how gases behave under changing conditions.

* Jacques Charles (1787): Charles's Law states that the volume of a gas is directly proportional to its absolute temperature, at constant pressure. This further expanded our understanding of how temperature affects gas volume.

* Gay-Lussac (1802): Gay-Lussac's Law states that the pressure of a gas is directly proportional to its absolute temperature, at constant volume. This completed the basic understanding of the relationship between temperature, pressure, and volume in gases.

These scientists built upon the work of others and laid the foundation for the ideal gas law, which combines all three laws and provides a more complete description of gas behavior.

It's important to note that the concept of gas expansion was likely understood even before the formal scientific investigations. People observed the expansion of air when heated, and the concept was likely used in various practical applications.

Therefore, it's more accurate to say that gas expansion was discovered gradually, with contributions from numerous individuals over time.