SCIENCE AND INDUSTRY. 
that was once the main object of strategy. Where sight fails to locate 
his batteries, sound is made to do its work. Bombing by airships and 
aeroplanes forms a new branch of tactics, as does also the offensive by 
gas attacks, and the corresponding defensive by gas masks. Add to 
these the use of submarines and the power of instantaneous communica- 
tion with each unit of a fleet wherever located, by means of wireless 
telegraphy, and we realize that it was no empty phrase to say that we 
had thus entered into a new era. 
Screncre or Expnosives. 
Take the subject of explosives, the basis of all modern warfare. The 
science of explosives is merely a study of the phenomena of combustion. 
An explosive is nothing other than a combustible substance that can 
burn rapidly without needing to be in communication with the external 
air. This may sound a very gentle definition of so terrifying a thing 
as an explosive, the very name of which suggests’ the possession of 
enormous force, under uncertain control. People are prepared to credit 
such a substance with secret stores of power wholly surpassing those 
that are possessed by other bodies. This is, however, far from being 
the case. It may surprise many, yet it is true. It may safely be said 
that there will never be an explosive which can give out nearly as much 
energy as an equal weight of coal or petroleum. Nitro-glycerine stands 
first of our most violent explosives, and yet the power it can generate 
is less than one-sixth of that given by the combustion of an equal weight 
of good coal. The difference in the effects in the two cases is due to 
the fact that the coal needs the oxygen of the air for its combustion. 
In nitro-glycerine the oxygen is to be found in the explosive itself. The 
explosive can therefore burn with rapidity, and within closed walls, and 
the hot gases thus generated, which are many hundred, or even thou- 
sand, times the volume of the explosive itself, gives the sudden pressure 
which bursts the containing walls, or, in a gun, drives out the projectile. 
The history of explosives was the history of the means of laying up this 
oxygen close by the side of the combustible. It was first accomplished 
in the invention of gunpowder. Here the combustible was a mixture 
of charcoal and sulphur, and the oxygen was contained in saltpetre, which 
is the best known member of a class of bodies that contain large quan- 
tities of oxygen, which is readily given off when they are heated to a 
high temperature in the presence of combustible matter. Gunpowder 
is a remarkable instance of successful invention. Whether it was 
Roger Bacon, or some unknown Chinese forerunner, who first devised 
it may be doubtful, but the ingredients were rightly chosen from the 
first, and the proportions have practically remained the same from early 
in the Fourteenth Century to the present day. It can boast of having 
satisfied all military requirements for five and a half centuries, and is 
even now very far from being superseded at the present day. 
Excellence in the explosive was attained in the case of gunpowder 
by grinding the materials to fine powder and thoroughly incorporating 
them. Thus each particle of combustible had its necessary oxygen close 
at hand, and when the combustion was started at some one point by a 
spark, it rushed through the mass with a speed sufficient to cause the 
explosion. Some 80 years ago, chemists discovered substances in which’ 
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