William C. McC. Lewis: Architect of the Collision Theory
William Cudmore McCullagh Lewis (1885–1956) was a foundational figure in 20th-century physical chemistry. While his name may not carry the household recognition of a Newton or a Curie, his intellectual fingerprints are all over the fundamental equations taught in every introductory chemistry classroom today. As the primary architect of the Collision Theory, Lewis provided the mathematical bridge that linked the chaotic movement of gas molecules to the predictable rates of chemical reactions.
1. Biography: From Belfast to the Liverpool Chair
William Lewis was born on June 29, 1885, in Belfast, Ireland. He was a product of the rigorous Scotch-Irish academic tradition, receiving his early education at the Royal Belfast Academical Institution before attending Queen’s College (now Queen’s University) Belfast.
After earning his M.A. in 1906, Lewis moved to University College London (UCL) to work under the titan of physical chemistry, Sir William Ramsay, and later F.G. Donnan. This period was transformative; UCL was then the epicenter of research into the kinetic theory of gases and thermodynamics. Lewis earned his Doctorate of Science (D.Sc.) in 1909 and spent a brief period in Germany—then the world leader in chemical research—studying under Luther at Leipzig.
In 1913, at the remarkably young age of 28, Lewis was appointed to the Brunner Chair of Physical Chemistry at the University of Liverpool. He would remain at Liverpool for the rest of his career, eventually becoming the Grant-Brunner Professor in 1937. His tenure was marked by a commitment to transforming chemistry from a descriptive science into a rigorous, mathematical discipline.
2. Major Contributions: The Birth of Collision Theory
The crowning achievement of Lewis’s career came in 1918 with the publication of his paper, "Studies in Catalysis, Part IX: The Calculation in Absolute Measure of Velocity Constants and Equilibrium Constants in Gaseous Systems."
The Collision Theory
Before Lewis, chemists knew that temperature increased reaction rates (the Arrhenius Equation), but they didn't fully understand why on a molecular level. Lewis applied the kinetic theory of gases to chemical reactions. He proposed that:
- Collision Frequency: For a reaction to occur, molecules must physically collide.
- Energy Threshold: Only collisions with a specific minimum kinetic energy (the activation energy) result in a reaction.
- Mathematical Precision: He derived a formula that allowed scientists to calculate the "rate constant" of a reaction based on the diameter of the molecules and their average velocity.
While the German chemist Max Trautz had reached similar conclusions independently in 1916, Lewis’s 1918 work was more comprehensive and accessible to the English-speaking scientific community, providing the first "absolute" calculation of reaction velocities.
The Radiation Hypothesis
Early in his career, Lewis was a proponent of the "Radiation Hypothesis," the idea that chemical reactions were triggered by the absorption of infrared radiation from the environment. While this theory was eventually proven incorrect (replaced by the collision and transition-state theories), Lewis’s rigorous defense of it spurred significant debate that helped refine the laws of photochemistry.
3. Notable Publications
Lewis was a prolific writer whose textbooks became the gold standard for a generation of students.
- A System of Physical Chemistry (1916): A three-volume masterwork. At the time, it was considered the most comprehensive textbook on the subject in the English language, meticulously synthesizing thermodynamics, kinetic theory, and quantum theory.
- Studies in Catalysis, Part IX (1918): Published in the Journal of the Chemical Society, this is his most influential research paper, laying out the mathematical framework for Collision Theory.
- The Quantum Theory (1918): One of the earliest attempts to integrate the burgeoning field of quantum mechanics into the teaching of physical chemistry.
4. Awards and Recognition
Lewis’s contributions were recognized by the highest echelons of British science:
- Fellow of the Royal Society (FRS): Elected in 1926, the most prestigious honor for a British scientist, in recognition of his work on reaction kinetics and thermodynamics.
- The Brunner Professorship: His appointment to this chair at such a young age was a testament to his rising-star status in the early 1910s.
- The Faraday Society: He was a prominent member and contributor to the Faraday Society, which was the premier venue for physical chemistry discussions in the UK.
5. Impact and Legacy
William Lewis’s legacy is found in the Lewis-Trautz Collision Theory. Every student who learns about "effective collisions" and the "pre-exponential factor" in kinetics is studying Lewis’s work.
Beyond his specific theories, Lewis played a pivotal role in the "Mathematicalization" of Chemistry. Before his era, chemistry was often seen as a branch of pharmacy or metallurgy. Lewis helped pivot the field toward physics, insisting that chemical behavior could be predicted using the laws of statistics and mechanics.
His "Liverpool School" of chemistry produced dozens of researchers who went on to lead departments across the British Empire, ensuring his pedagogical approach survived long after his retirement in 1948.
6. Collaborations and Mentorship
- F.G. Donnan: Lewis was a protégé of Donnan (famous for the Donnan Equilibrium). Their relationship established a lineage of rigorous physical chemistry in Britain.
- Max Trautz: Though they did not collaborate directly (due in part to the communication barriers of World War I), their names are forever linked in history as the co-discoverers of collision theory.
- The Liverpool Group: Lewis was known for fostering a collaborative atmosphere at Liverpool, working closely with researchers like Griffith and McKeown on complex problems in solubility and catalysis.
7. Lesser-Known Facts
- War Work: During World War I, Lewis’s expertise was diverted to the war effort. He conducted vital research on the physical chemistry of explosives and the stabilization of emulsions, which had direct applications in both munitions and field medicine.
- The "Failed" Theory: Lewis is a rare example of a scientist whose "greatest mistake"—the Radiation Hypothesis—is still respected. His failure to prove it led directly to the realization that molecular collisions, not ambient light, were the primary drivers of gas-phase reactions.
- A Quiet Life: Despite his massive influence, Lewis was known to be a modest and somewhat reserved man, preferring the laboratory and the lecture hall to the public spotlight. He remained a bachelor for much of his life, dedicated almost entirely to his "System of Physical Chemistry."
Conclusion
William C. McC. Lewis was the bridge between the classical chemistry of the 19th century and the quantum, kinetic-driven chemistry of the 20th. By asking exactly how two molecules meet and transform, he turned the invisible dance of atoms into a measurable, predictable science.