Free The Magic of Reality by Richard Dawkins Page B

gas molecules charge about faster.
    A liquid is like a gas in that its molecules move around or ‘flow’ (that’s why both are called ‘fluids’, while solids aren’t). But the molecules in a liquid are much closer to each other than the molecules in a gas. If you put a gas into a sealed tank, it fills every nook and cranny of the tank up to the top. The volume of gas rapidly expands to fill the whole tank. A liquid also fills every nook and cranny, but only up to a certain level. A given amount of liquid, unlike the same amount of gas, keeps a fixed volume, and gravity pulls it downwards, so it fills only as much as it needs of the tank, from the bottom upwards. That’s because the molecules of a liquid stay close to each other. But, unlike those of a solid, they do slide around over each other, which is why a liquid behaves as a fluid.
    A solid doesn’t even try to fill the tank – it just retains its shape. That’s because the molecules of a solid don’t slide around over each other like those of a liquid, but stay in (roughly) the same positions relative to their neighbours. ‘Roughly’ because even in a solid the molecules do sort of jiggle about (faster at higher temperatures): they just don’t move far enough from their position in the crystal ‘parade’ to affect its shape.
    Sometimes a liquid is ‘viscous’, like treacle. A viscous liquid flows, but so slowly that, although a very viscous liquid eventually fills the bottom part of the tank, it takes a long time to do so. Some liquids are so viscous – flow so slowly – that they might as well be solid. Substances of this kind behave like solids, even though they’re not made of crystals.
    Solid, liquid and gas are the names we give to the three common ‘phases’ of matter. Many substances are capable of being all three, at different temperatures. On Earth, methane is a gas (it’s often called ‘marsh gas’, because it bubbles up from marshes, and sometimes it catches fire and we see it lit up as eerie ‘will o’ the wisps’). But on a large, very cold moon of the planet Saturn called Titan there are lakes of liquid methane. If a planet were colder still, it might have ‘rocks’ of frozen methane. We think of mercury as a liquid, but that just means it’s liquid at ordinary temperatures on Earth. Mercury is a solid metal if you leave it outside in the Arctic winter. Iron is a liquid if you heat it to a high enough temperature. Indeed, around the deep centre of the Earth is a sea of liquid iron mixed with liquid nickel. For all I know, there may be very hot planets with oceans of liquid iron at the surface, and perhaps strange creatures swimming in them, although I doubt that. By our standards, the freezing point of iron is rather hot, so at the surface of the Earth we usually encounter it as ‘iron – cold iron’ (Google it. It’s from the poet Rudyard Kipling), and the freezing point of mercury is rather cold, so we usually encounter it as ‘quicksilver’. At the other end of the temperature scale, both mercury and iron become gases if you heat them enough.
    Inside the atom
    When we were imagining cutting matter into the smallest possible pieces at the beginning of this chapter, we stopped at the atom. An atom of lead is the smallest object that still deserves to be called lead. But can you really not cut an atom any further? And would an atom of lead actually look like a tiny little chip of lead? No, it wouldn’t look like a tiny piece of lead. It wouldn’t look like anything. That’s because an atom is too small to be seen, even with a powerful microscope. And yes, you can cut an atom into even smaller pieces – but what you then get is no longer the same element, for reasons we shall soon see. What is more, this is very difficult to do, and it releases an alarming quantity of energy. That is why, for some people, the phrase ‘splitting the atom’ has such an ominous ring to it. It was first done by the great New Zealand