Such a transformation that takes place without a change in temperature is said to be isothermal. The pressure is reduced at a rate slow enough that heat from the environment is able to keep the jar and its contents at nearly room temperature. The marshmallow-vacuum experiment shown above is an example of a 'slow' process. Since the gas needs to be in thermal equilibrium with its environment (or some other heat reservoir) to maintain an even temperature, the pressure-volume relationship normally applies only to 'slow' processes. Boyle neglected to mention it, but the data he used to derive his law were most likely collected during a period in which the temperature did not experience any significant change. Mariotte added the important provision that temperature remain constant. In Great Britain, America, Australia, the West Indies and other remnants of the British Empire it is called Boyle's law, while in Continental Europe and other places it is called Mariotte's law. This correlation was discovered independently by Robert Boyle (1627–1691) of Ireland in 1662 and Edme Mariotte (1620–1684) of France in 1676. The pressure of a gas is inversely proportional to its volume when temperature is constant. This illustrates a fundamental, yet important, property of gases. Since the vacuum pump pulls on the marshmallows hard enough to burst some of the air bubbles, they are actually a bit smaller and more shriveled at the end of this experiment. Break the seal on their container and they shrink during the return to normal atmospheric pressure. When placed in a vacuum pump, they expand as the pressure decreases. Marshmallows are a frozen foam and are mostly air by volume. Sugar makes them sweet, air makes them fluffy, and gelatin makes them elastic. Marshmallows are a mixture of sugar, air, and gelatin. The volume of a marshmallow increases as the pressure on it decreases - and vice versa. Let's try the following experiment using equipment that might be found in your kitchen. What happens to the volume of a gas as the pressure on it changes. However, since the laws are known by different names in different countries and (more importantly) since I can never remember who gets credit for which law without referring to notes, I will not follow this convention. For this reason, many students are taught the three most important gas laws by the names of their discoverers. And yet someone had to notice these relationships and write them down. It's hard to believe that there was ever a time when they weren't understood. The gas laws are a set of intuitively obvious statements to most everyone in the Western world today.

Ideal gas law