how molecular motions produced the observed gross behavior, he would have achieved both a deeper understanding of the phenomena and have provided strong new evidence for the very existence of atoms and molecules, which was at the time—in the mid-19th century—a contentious conviction in some circles.

The idea that a gas’s properties depended on the motion of its constituent molecules was not new, though. It was known as the kinetic theory of gases, originally articulated in 1738 by our old friend Daniel Bernoulli, who explained the gas laws with a crude picture of molecules modeled as billiard balls. But as the science historian Stephen Brush has noted, Bernoulli’s theory “was a century ahead of its time.”14 Bernoulli’s idea was based on the (correct) notion that heat is merely the motion of molecules, but in his day most physicists believed heat to be some sort of fluid substance (called caloric). By the 1850s, though, the kinetic theory was a ripe topic for physicists to study, as the laws of thermodynamics—constituting the correct theory of heat—were arriving on the scene.

One of the major pioneers of thermodynamics was the German physicist Rudolf Clausius. In an 1857 paper, Clausius presented a comprehensive view on the nature of heat as molecular motion. He described how the pressure of a gas was related to the motion of molecules as they impinged on the walls of their container. And any given molecule was constantly battered by collisions with other molecules, so its behavior reflected the influences of such impacts (just as a person’s choices reflect the influences of the countless social pressures that Quetelet had described). In his approach, Clausius emphasized the importance of the average velocity of the molecules, and in an 1858 paper introduced the important notion of the average distance that molecules traveled between collisions (a distance labeled the “mean free path”).

In 1859, Maxwell entered the molecular motion arena, exploring the interplay of gas molecules and their resulting velocities a little more deeply. In his approach, Maxwell applied the sort of statistical thinking that Quetelet had promoted.

Maxwell had probably first encountered Quetelet in an article

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