Then again some of the most important explosives are solid or semi-solid, and yet they require to be mixed in order to form the various "powders" in use by our gunners. The best way to bring about this mixture is to dissolve the two components in alcohol, thereby forming them both into liquids which can be readily mixed. Afterwards the alcohol evaporates; indeed, one of its great virtues for this and similar purposes is that it quietly takes itself off when it has done its work like a very well-drilled servant. What then is this precious liquid and how is it produced? In order to answer that question it is necessary first to state that there are a whole family of substances called "alcohols," all of which are composed of carbon, hydrogen and oxygen in certain proportions. There are also a number of kindred substances also, not exactly brothers but first cousins, so to speak, which because of their resemblance to this important family have names terminating in "ol." They owe their existence to the wonderful behaviour of the atoms of carbon. In order to obtain some sort of system whereby the various combinations of carbon can be simply explained chemists picture each carbon atom as being armed with four little links or hooks with which it is able to grapple, as it were, and hold on to other atoms. Each hydrogen atom, likewise, has its hook, but only one instead of four. Now it is easy to picture to ourselves an atom of carbon in the middle with its hooks pointing out north, south, east and west with a hydrogen atom linked on to each. That gives us a picture of the molecule of Methane, the gas which forms the chief constituent of coal gas such as we burn in our homes. Methane is also given off by petroleum and it is the cause of the explosions in coal mines, being known to the miners as "firedamp." It is the first of a long series of substances which the chemist called paraffins. The first, as you see, consists of one of carbon and four of hydrogen. Add another of carbon and two more of hydrogen and you get the second "Ethane." Add the same again and you get the third, Propane, and so on until you can reach a substance consisting of thirty-five parts of carbon and seventy-two parts of hydrogen. All we need trouble about, however, is the first two, Methane and Ethane.