As my AP Chemistry teacher says, Amontons is the guy "nobody knows about" when it comes to gas laws. He's forgotten amongst the laws of Boyle, Charles, and Avogadro. Which is really quite odd, because his law is sort of important (though rather intuitive, too).

Amontons's law states that in an ideal gas, when volume and number of moles are held constant, the pressure and temperature of a gas are directly related.


Or, in another form:

P1 / T1 = P2 / T2

Considering the ideal gas law, this one makes perfect logical sense. And I have my own layman's explanation for why this is so. Pressure is related to how often the various molecules of a gas strike the container in which it is present. Temperature is directly proportional to the average kinetic energy of the molecules of a gas (it's true). When temperature is increased, average kinetic energy increases; thus average velocity of the gas molecules also increases (Kinetic energy = mass * speed^2 / 2). So, the particles are zipping about the container a bit faster now. Since you still have the same number of molecules and the same volume (by definition of the law), but faster-moving molecules, the molecules will collide with the container more often, thus increasing pressure.

Near the end of the 17th century, Amontons built a thermometer based on this relationship. In 1779, Joseph Lambert used experimental data that demonstrated this proportionality, and derived a value for absolute zero at about -270 °C (though, with more accurate measurements, it is really about -273.15 °C). The apparatus used in the experiment was a pressure gauge, attached to a hollow metal sphere filled with gas. This design keeps the amount of gas present and the volume of the gas constant. The sphere was submerged in liquids of various known temperatures, and the gas was allowed to reach the same temperature as the liquid. The pressure shown by the pressure gauge was then recorded after each liquid. Thus, using the idea of Amontons's law and some extrapolation, Joseph Lambert arrived at the first definition for a value of abolute zero. (fun trivia: in 1702, Amontons was the first to propose the idea of an absolute zero temperature, but lacked accurate and precise thermometers to find any real value for it.)

So there's both some science and some history for you.

Sources: (31 December, 2005)

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