Though he’ll be forever remembered as the father of electromagnetism, one of James Clerk Maxwell’s most famous lectures had nothing at all to do with this subject. In 1873, he addressed the British Association for the Advancement of Science on another topic close to his heart. He spoke of “molecules,” though he was referring more generally to the concept that gases are composed of small particles that are busily whizzing about.
He mentioned that the air in the lecture hall was filled with molecules traveling about in all directions at a speed of about 17 miles per minute. Maxwell and his contemporaries understood that the temperature and the pressure of the air about them were directly proportional to the cruising speed of the gas particles. There are actually about 1×1023 particles within the volume of, say, a beach ball. Since the speed of these will vary somewhat over a specific range, it is more accurate to say that the ambient temperature and pressure are determined by the average speed of all those particles cruising about.
The general relationship between particle speed, temperature, and pressure was the subject a branch of physics in and of itself. Thermodynamics, as it is known, can be classified as the third and final pillar of classical physics. As evidenced by Maxwell’s famous lecture, at its very heart are the small particles that make up the air all around us. It is to these that our story now turns.
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