Who is a frenchman whose interest in hot air balloonl led to an expression relating the volume of balloons to their temperature?

The Frenchman you're referring to is Jacques Charles, a physicist and inventor. He is known for Charles's Law, which describes how gas volume is directly proportional to its temperature when pressure is constant. His experiments with hot air balloons in the late 18th century demonstrated this relationship, leading to the expression linking the volume of gases to temperature.