Knudsen's gauge

Thermodynamics is the 19th century equivalent of the grand unified theory of the universe. Starting with Count Rumford and Sadi Carnot’s speculations about the heat produced by the boring of cannons, by the middle of the century a clutch of scientists had turned it into a robust conceptual framework that used energy and its distribution to join the dots between every chemical and physical phenomenon.

Yet the framework was predicated on a series of crucial assumptions about the nature of matter: that it consisted of discrete particles that behaved as independent entities. For some physical scientists this was too much; famously Ernst Mach and Wilhelm Ostwald refused to accept it. Sure the maths worked, they argued, but these supposed ‘atoms’ were nothing more than a convenient fiction. The idea was largely left by the wayside until the end of the 19^th century, when its re-emerged thanks to work by a small number of scientists who began to think deeply about the nature of gases and about vacuum.