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
The word mine in its military sense originally meant just the same as it does in the ordinary way, but like many other words it has got twisted into new uses the connection of which with the original meaning is very obscure. One of the most striking of these verbal puzzles is the submarine mine. There seems at first sight not the remotest connection between the floating barrel of explosives concealed beneath the water and what we ordinarily call a mine. The explanation of this is that the term has acquired this meaning after passing through a series of stages.
When soldiers "mine" for the purpose of blowing up their enemies they dig a hole in the ground, and conceal therein a quantity of explosives so arranged that they blow up when the enemy pass over or near. The operation of digging the hole in the earth is clearly akin to the work of the miner and so such is quite appropriately called a "mine."
The hole may be dug from the surface downwards, the marks of excavation being afterwards covered up and obliterated as much as possible. In other cases the hole may be a tunnel starting from a trench and