Let us think of Nature as a builder, making all that we see out of atoms of a limited number of kinds, just as the builder of a house constructs it out of so many different kinds of things: bricks, slates, planks, panes of glass, and so on.

Broadly speaking, the discovery of X-rays has increased the keenness of our vision ten thousand times, and we can now 'see' the individual atoms and molecules.

In a gas, motion has the upper hand; the atoms are moving so fast that they have no time to enter into any sort of combination with each other: occasionally, atom must meet atom and, so to speak, each hold out vain hands to the other, but the pace is too great and, in a moment, they are far away from each other again.

The infinite variety in the properties of the solid materials we find in the world is really the expression of the infinite variety of the ways in which the atoms and molecules can be tied together, and of the strength of those ties.

The atoms may be compared to the letters of the alphabet, which can be put together into innumerable ways to form words. So the atoms are combined in equal variety to form what are called molecules.

The difference between a gas and a liquid is that in the former, the atoms and molecules move to and fro in an independent existence, whereas in the latter, they are always in touch with one another, though they are changing partners continually.