The reason why a gas is so active is because it is so egoistic.  Psychologically interpreted, a gas consists of particles having the utmost aversion to one another.  Each tries to get as far away from every other as it can.  There is no cohesive force; no attractive impulse; nothing to draw them together except the all too feeble power of gravitation.  The hotter they get the more they try to disperse and so the gas expands.  The gas represents the extreme of individualism as steel represents the extreme of collectivism.  The combination of the two works wonders.  A hot gas in a steel cylinder is the most powerful agency known to man, and by means of it he accomplishes his greatest achievements in peace or war time.

The projectile is thrown from the gun by the expansive force of the gases released from the powder and when it reaches its destination it is blown to pieces by the same force.  This is the end of it if it is a shell of the old-fashioned sort, for the gases of combustion mingle harmlessly with the air of which they are normal constituents.  But if it is a poison gas shell each molecule as it is released goes off straight into the air with a speed twice that of the cannon ball and carries death with it.  A man may be hit by a heavy piece of lead or iron and still survive, but an unweighable amount of lethal gas may be fatal to him.

Most of the novelties of the war were merely extensions of what was already known.  To increase the caliber of a cannon from 38 to 42 centimeters or its range from 30 to 75 miles does indeed make necessary a decided change in tactics, but it is not comparable to the revolution effected by the introduction of new weapons of unprecedented power such as airplanes, submarines, tanks, high explosives or poison gas.  If any army had been as well equipped with these in the beginning as all armies were at the end it might easily have won the war.  That is to say, if the general staff of any of the powers had had the foresight and confidence to develop and practise these modes of warfare on a large scale in advance it would have been irresistible against an enemy unprepared to meet them.  But no military genius appeared on either side with sufficient courage and imagination to work out such schemes in secret before trying them out on a small scale in the open.  Consequently the enemy had fair warning and ample time to learn how to meet them and methods of defense developed concurrently with methods of attack.  For instance, consider the motor fortresses to which Ludendorff ascribes his defeat.  The British first sent out a few clumsy tanks against the German lines.  Then they set about making a lot of stronger and livelier ones, but by the time these were ready the Germans had field guns to smash them and chain fences with concrete posts to stop them.  On the other hand, if the Germans had followed up their advantage when they first set the cloud of chlorine floating over the battlefield of Ypres they might have won the war in the spring of 1915 instead of losing it in the fall of 1918.  For the British were unprepared and unprotected against the silent death that swept down upon them on the 22nd of April, 1915.  What happened then is best told by Sir Arthur Conan Doyle in his “History of the Great War.”