"Because those letters are painted on the ogives of the shells. The 'T' shells are filled with a very dense gas, which disperses slowly. After a storm of these shells has fallen, the air is unbreathable for an hour or sometimes two, according to the dampness of the weather. The 'K' shells are filled with a more powerful spasm-gas, virulent in its effects, but which disperses rapidly. "The first is used in curtain fire, when the Germans expect to be assaulted. A steady dropping bombardment of 'T' shells makes a gas-filled zone. Charging troops have to wear gas masks, for they must pass through it. Defending troops do not need to wear masks, and, as you know yourself, a man is twice as quick and agile without a mask. "The second, or 'K' shell, is used when the enemy plans to make the assault. You can't see the shells coming, there is no evidence of any change in the enemy's lines which can be reported by an aëroplane. No one knows when the German artillery has received orders to change from high explosive or shrapnel to gas shells, when, suddenly, all along the line, there drops a concerted hail of gas shells, and in ten seconds half the men in the first line trenches are gassed. It takes about twenty seconds to put on a gas-mask properly. It is a horrible, vicious, and cowardly way of making war." "But don't we use it, too?" "We haven't yet," the veteran answered, "but we shall have to begin soon, in self-defense.[23] Then the Boches will be sorry that they began, for their own atrocious cruelty will return on their own heads. But we have a new invention, too, which is gaining us more ground than we lost by the poison gas." "You mean the tanks?" "Yes." "I'd like to see a tank in action," said Horace, eagerly. "But I suppose we won't have them, here." [Illustration: _Courtesy of "L'Illustration."_ THE APPROACH OF DOOM. British tank, first appearing at Flers (September 15, 1916) which drove the German Army into a panic of unreasoning terror.] "We shall," the veteran replied, "and soon. We shall be compelled to use them. The night before last, the Germans started using liquid fire on our lines. That's a wicked thing, too. From what I hear, it is a mixture of gasoline, paraffine and tar, forced out by compressed nitrogen and ignited at the point of a long tube. It throws a jet of fire twenty or even thirty yards.[24] It burns a man to a crisp where he stands. No gas-mask will stop that." "And the tanks don't mind it?" "A tank minds nothing," was the answer. That very night, Horace learned what a tank looked like.