In the 17th century, scientists understood three phases of matter—solids, liquids and gases (the discovery of plasma, the fourth phase of matter, lay centuries in the future). Back then, solids and liquids were much harder to work with than gases because changes in solids and liquids were difficult to measure with the equipment of the time. So many experimentalists played around with gases to try to deduce fundamental physical laws. Robert Boyle was perhaps the first great experimentalist, and was responsible for what we now consider to be the essence of experimentation: vary one or more parameter, and see how other parameters change in response. It may seem obvious in retrospect, but hindsight, as the physicist Leo Szilard once remarked, is notably more accurate than foresight.
Boyle discovered the relationship between the pressure and volume of a gas, and a century later, the French scientists Jacques Charles and Joseph Gay-Lussac discovered the relationship between volume and temperature. This discovery was not simply a matter of donning a traditional white lab jacket (which hadn’t yet been invented) and performing a few measurements in comfortable surroundings. To obtain the required data, Gay-Lussac took a hot-air balloon to an altitude of 23,000 feet, possibly a world record at the time. The results of Boyle, Charles and Gay-Lussac could be combined to show that in a fixed quantity of a gas, temperature was proportional to the product of pressure and volume. The constant of proportionality is known as the ideal gas constant. Edited from the ideal gas constant