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.
— William Henry Bragg
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.
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 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 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.
Sound is a movement which is handed on from atom to atom in a gas through which the sound is passing, just as a chain of workers pass buckets of water to a fire. The quicker the workers move their hands and arms, the quicker the water moves.
The chemist, whose science is immediately concerned with the combinations of atoms, has rarely found it necessary to discuss their shapes, and gives them no particular forms in his diagrams. That does not mean that the shapes are unimportant, but rather that the older methods could not define them.
When a liquid boils, the temperature has been raised to such a pitch that the evaporating molecules are sufficient in number and speed to lift off the air from the surface of the liquid and push it back en masse.
Whenever we warm our hands by the fire, we allow the energy radiated by the fire to quicken up the movements of the atoms of which the hands are composed. When we cool any substance, we check those movements.
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.
