We have pictured to ourselves the molecule of methane: let us do the same with ethane. Imagine two carbon atoms side by side linked together or hand in hand. Each will be using one of its hooks to grasp one hook of its brother atom. Hence each will have three hooks to spare on to which we can hook a hydrogen atom. Thus we get two of carbon and six of hydrogen neatly and prettily linked up together. The atoms form an interesting little pattern and to build up the various paraffin molecules with a pencil and paper has all the attractions of a puzzle or game. All you have to do is to add a fresh atom of carbon alongside the others and then attach an atom of hydrogen to each available unused hook. If you care to try this you will get the whole series, each one having one atom of carbon and two of hydrogen more than its predecessor. If you mix together a quantity of methane and an equal quantity of chlorine, which I have shown you in another chapter how to get from common salt, a change takes place, for in each molecule of methane one hydrogen atom becomes detached and an atom of chlorine takes its place. How or why this change occurs we do not know. It is a fact that the chlorine has this power to oust the hydrogen and there we must leave it, for the present at any rate. The substance so formed is called methyl chloride. In another chapter reference has been made to that substance which is made from common salt and which is so important in so many manufactures called caustic soda. If we bring some of it into contact with the methyl chloride the chlorine is punished for its rudeness in displacing the hydrogen; it is paid back in its own coin, for it is in turn displaced not this time by a single atom but by a little partnership called "hydroxyl" one atom of hydrogen and one of oxygen acting together. We can again form a neat little picture of what happens. The oxygen atom has two hooks, one of which it gives to its friend the hydrogen atom and thus they go about hand-in-hand, the oxygen having one unused hook with which to hook on to something else. In this case it hooks on to that particular hook from which it pushes the chlorine. We have thus seen two changes take place. First, the hydrogen is displaced by the chlorine: then the chlorine is turned out and its place taken by the hydroxyl. And during both these changes the central carbon atom and its three hydrogen partners have remained unaffected. Those four atoms are called the methyl group, and a methyl group combined with a hydroxyl group forms _methyl alcohol_. Similar changes can be brought about with Ethane as with Methane, and in them the two carbon atoms and the five hydrogen remain unchanged, whence they too are regarded as a group, the Ethyl group, and an ethyl group hooked on to a hydroxyl group gives us a molecule of _ethyl alcohol_.