We can rest assured : atoms really do exist. However, certain picky purists could ask questions about the signification of this expression : what should we understand when we speak about the "real existence" of something whose size in measured in cento-millions of a centimeter and whose behaviour is governed by laws (that of quantum mechanics) so fundamentally distinct from those we have been taught for centuries in "classical" physics - ever since the times of Galileo and Newton ? What is more, the reality of atoms and molecules gave rise, at the turn of the century (around 1900), to a controversy sometimes quite ferocious, concerning the very phenomenons and concepts treated in this book. And so, Paul Pascal, who was an exceptional chemist, used to tell the story of how, in 1905, a young student and candidate for the General Chemisty certificate at the Sorbonne, he was assigned to a professor who refused the existence of atoms and forbade others to even consider the possibility. Worse still, the Viennese Ludwig Bolztmann, genius promoter of mechanical statistics, was pushed to suicide in 1906 by the sarcasms of his detractors. Several months later in the same year, a Frenchman, Jean Perrin, proved the irrefutable existence of atoms and the validity of mechanical statistics. It would only be fair to recognize that the anti-atomnists, in order to be so virulent and forceful in their claims, must have had a theory of physics extremely coherent and efficient - thermodynamics, of which Bernard Diu also speaks of in great length - and thus apparently had no need for atoms. After having first of all clarified the theory of physics, what is physics and what isn't, Bernard Diu explains in a very simple way, the implications and the eventual oppositions, between thermodynamics and statistical mechanics.

Bernard Diu, a graduate of the Ecole Normale Supérieur, is a professor at the University of Paris VII.