Boyle's experiments all were carried out at constant temperature. A century later, Jacques Charles, in France, studied what happens to the volume of a gas when the temperature is changed and the external pressure is kept constant. This is the problem of heating or cooling a balloon full of air, with a fixed outside pressure exerted by the surroundings. In every gas he studied, Charles observed a steady increase in volume with an increase in temperature.

Translating his data into modern units, he found that for every degree Celsius, or centigrade, rise in temperature, the gas volume increased by 1/273 of its volume at 0. This is easier to understand from the graph of volume versus temperature shown above. Within the observed range of temperatures, the plot is a straight line. If we extend this straight line back to zero volume, it crosses the temperature axis at -273.15. (For simplicity in the discussion that follows, we often shall use -273.)