Thermodynamics

The seventh entry is isotherme, and an English reader will meet it far more often in a supermarket than in a textbook. In the physics it means what « isothermal » means: a change that holds the temperature constant, which in practice needs heat to flow freely in or out while a gas is squeezed or allowed to expand. In the shops it is printed on objects built to do the opposite. Un sac isotherme is a cool bag, une bouteille isotherme a vacuum flask, un camion isotherme an insulated lorry, and every one of them works by stopping heat from crossing its walls, which is precisely the definition of the entry just above, adiabatique. The everyday word looks at the contents, whose temperature stays put; the physics looks at the process and at the exchanges that keep it level. A third use turns up in every mountain weather forecast: l’isotherme zéro degré, now a feminine noun, is the height at which the air reaches freezing point, what British forecasters call « the freezing level ». One word, three jobs, and only the sentence around it decides which.