Introduction

Suppose our microscopes had no limit to their power, so that we could examine a drop of water at any magnification we like. What would we find if the drop were enlarged a million or more times? Would we still see a clear, structureless liquid? If not, what else?

The answer is that, on a very small scale of size, our drop of water consists of billions of tiny separate particles. Indeed, all matter does, whether in the form of a solid, a liquid or a gas. This much was suspected over 2000 years ago in ancient Greece. Modern science has not only confirmed this suspicion but extended it: the particles that make up all matter are in constant random motion, and the kinetic energy of this motion is what constitutes heat. In everyday life, matter shows no direct sign of either the particles or their motion. However, plenty of indirect signs support this picture, and we shall consider some of them in this chapter.