At first the two new substances are not apparent, for they remain together in one liquid. The addition, however, of some brine causes the change to become obvious for something in the liquid turns solid, so that it can be easily taken away from the rest. That solid is nothing else than soap. It remained dissolved in the water which forms part of the liquid until the salt was put in, but as it will not dissolve in salt water, as you will discover if you attempt to wash in sea water, it separates out as soon as the salt is added. But still a liquid remains: what can that be? It is mainly salt water and glycerine, that sticky stuff which in peace times we put on our hands if they get sore in winter, or take, in a little water, to soothe a sore throat. That it has other and very different uses was brought home to me when, during the war, I tried to buy some at a chemist's, only to learn that it could not be sold except in cases of extreme need under the orders of a doctor. The mixed liquid is distilled with the result that the water is driven off and the salt deposited, which with other minor purifying processes gives the pure glycerine. The next step takes us to the explosives factory, where the glycerine is mixed with sulphuric and nitric acids. Now glycerine, as you will have observed, comes from the animal or vegetable sources and therefore is one of those substances known as "organic," and, like many other of the organic compounds, it consists of carbon, hydrogen and oxygen. Nature has a marvellous way of combining these same three things together in many various ways to form many widely different substances and if, to such a compound, we can add a little nitrogen, we usually get an explosive. Thus, the glycerine, with some nitrogen from the nitric acid, becomes nitro-glycerine, a most ferocious and excitable explosive, the basis of several of those explosives without which warfare as we know it to-day would be impossible. CHAPTER II GUNPOWDER AND ITS MODERN EQUIVALENTS The origin of gunpowder appears to be lost in antiquity. At all events it has been in use for many centuries and is still made in many countries. Most boys have tried to make it at some time or other and with varying degrees of success. Such experiments generally lead to a glorious blaze, a delightfully horrid smell and no harm to anyone, the experimenter owing his safety to his invariable lack of complete success, for although other and better explosives have superseded it for many purposes it is capable of doing a lot of harm when it is well made.