The Science of Discworld Revised Edition

or wet. Sulphur, for example, was combustible (though not actually
hot
, you understand) and salt was incombustible.
    By 1661 Robert Boyle had sorted out two important distinctions, putting them into his book
The Sceptical Chymist
. The first distinction was between a chemical compound and a mixture. A mixture is just different things, well, mixed up. A compound is all the same stuff,
but
whatever that stuff is, it can be persuaded to come apart into components that are other kinds of stuff – provided you heat it, pour acid on it, or find some other effective treatment. What you can’t do is sort through it and find a different bit; for a mixture you can, although you might need very good eyesight and tiny fingers. The second distinction was between compounds and elements. An element really
is
one kind of stuff: you can’t separate it into different components.
    Sulphur is an element. Salt, we now know, is a compound made by combining (not just mixing) the two elements sodium (a soft, inflammable metal) and chlorine (a toxic gas). Water is a compound, made from hydrogen and oxygen (both gases). Air is a mixture, containing various gases such as oxygen (an element), nitrogen (also an element), and carbon dioxide (a combination of carbon and oxygen). Earth is a very complicated mixture and the mix varies from place to place. Fire isn’t a substance at all, but a process involving hot gases.
    It took a while to sort all this out, but by 1789 Antoine Lavoisier had come up with a list of 33 elements that were a reasonable selection of the ones we use today. He made a few understandable mistakes, and he included both light and heat as elements, but his approach was systematic and careful. Today we know of 113 distinct elements. A few of these are artificially produced, and several of those have existed on Earth only for the tiniest fraction of a second, but most elements on the list can be dug up, extracted from the sea or separated from the air around us. And apart from a few more artificially produced elements that it might
just
be possible to make in future, today’s list is almost certainly
complete
.
    It took another while for us to get that far. The art of alchemy slowly gave way to the science of chemistry. Gradually the list of accepted elements grew; occasionally it shrunk when people realized that a previously supposed element was actually a compound, such as Lavoisier’s lime, now known to be made from the elements calcium and oxygen. The one thing that didn’t change was the only thing the Greeks had got right: each element was a unique individual with its own characteristic properties. Density; whether it was solid, liquid, or gas at room temperature and normal atmospheric pressure; melting point if it was solid – for each element, these quantities had definite, unvarying values. It is the same on Discworld, with its to our eyes bizarre elements such as chelonium (for making world-bearing turtles), elephantigen (ditto elephants), and narrativium – a hugely important ‘element’ not just for Discworld, but for understanding our own world too. The characteristic feature of narrativium is that it makes
stories
hang together. The human mind loves a good dose of narrativium.
    In this universe, we began to understand why elements were unique individuals, and what distinguished them from compounds. Again the glimmerings of the right idea go back to the Greeks, with Democritus ’ suggestion that all matter is made from tiny indivisible particles, which he called
atoms
(Greek for ‘not divisible’). It is unclear whether anybody, even Democritus, actually believed this in Greek times – it may just have been a clever debating point. Boyle revived the idea, suggesting that each element corresponds to a single kind of atom, whereas compounds are combinations of different kinds of atoms. So the element oxygen is made from oxygen atoms and nothing else, the element hydrogen is made from hydrogen atoms and nothing else, but the compound water is
not
made from water atoms and nothing else, it is made from atoms of hydrogen and atoms of oxygen.
    By 1807, one of the most significant steps in the development of both chemistry and physics had taken place. The Englishman John Dalton had found a way to bring a degree of order to the different atoms that made up the elements, and to transfer some of that order to compounds too. His predecessors had noticed that when elements combine together to form compounds,