For a moment let us turn back to the commencement of the chapter to the subject of methane. When mixed with chlorine, it will be remembered, one hydrogen atom gave place to a chlorine atom. If the process be repeated another hydrogen atom will be displaced in the same way, while a further repetition will result in the removal of a third, when there will be a carbon atom in the centre with three chlorine and one hydrogen hooked on to it. With that picture in your mind's eye you will be contemplating the molecule of that wonderful and beneficent substance, chloroform. When we think of the numberless operations which have been carried out by the surgeons in the course of this last war we realize a little how great is the total sum of pain and suffering which has been saved through the agency of this substance, this simple neat little arrangement of five tiny atoms. Now that again is obtained in manufacture from alcohol. Alcohol, bleaching powder and water are mixed and then distilled, by which of course is meant that the mixture is evaporated by heat and the vapour collected and cooled back into liquid again. The liquid so obtained is chloroform. Hardly less important than this, in our military hospitals, is ether, to which reference has already been made. It, too, is manufactured from alcohol. The alcohol, together with sulphuric acid, is placed in a still and heated, the vapour given off being led to another vessel and there condensed. The liquid thus obtained is ether and so long as the supply of fresh alcohol is kept up the production of ether goes on continuously. The sulphuric acid does not disappear and so does not need to be replaced, from which it would appear as if it might just as well not be there, but that is not the case. It plays the part of what is called a "catalyst," one of the curiosities of chemistry. There are many instances in which two things will combine only in the presence of a third which appears to be itself unaffected. This third substance is a catalyst. It reminds one of the clergyman at a wedding who unites others but remains unchanged himself. In conclusion, one may mention that many of the medicines with which our injured men were coaxed back to health and strength owe their existence to alcohol, for many drugs are obtained from vegetable substances by dissolving out a part of the herb with alcohol. Thus, as a drink, it is unquestionably very harmful. Indeed, in that way it probably kills more people per year than its use in the manufacture of explosives caused in the worst year of the war. Yet it also furnishes chloroform, ether and medicinal drugs and performs a whole host of useful services to mankind. Finally, if oil and coal should ever run short it is quite prepared to run our engines for us. Truly it is a wonderful substance. CHAPTER V MINES, SUBMARINE AND SUBTERRANEAN