"The newest kind that we're using has a tin tube three inches long and an inch in diameter, prolonged on the exterior by a rubber appendix in which there is a valve opening outward. The valve cannot open inward at all. So, when poison gas is seen coming, you can put on your mask and take the tube in your teeth. You can't breathe through your mouth, then, because the valve in the pipe won't open inward, and none of the poison gas can get in. You breathe in through the nose and breathe out through the mouth."[22] "It's awfully uncomfortable," said Horace; "they make me go around with a gas mask in my pocket, but every time I put it on for a few minutes, I'm glad enough to take it off again." The veteran shook his head. "That's foolish," he said, "because you need to become accustomed to wearing it. Practice a little bit every day. If you don't, and suddenly find yourself in the middle of a gas cloud, you won't be able to stand it more than five minutes. You'll feel that you're choking for air. So you slip it off, just for a moment's relief, the green horror catches at your throat, and you're done." "But, as you said yourself," protested the boy, "a cloud of gas passes over, and then it's gone." "I said it used to be that way," the sergeant-major answered, "but it's not that way any more. The Germans don't send their gas from big fixed gasometers now; they have tanks which a man can carry on his back and from which the gas is jetted by compressed air. Infantry, with gas-masks on, can come right up behind the men carrying the gas tanks and, just as soon as the heavy poison fumes begin to fill the trenches, they charge." "Isn't there any way of stopping it?" "Only with a fearful amount of trouble and enormous expense. Poison gas, being heavier than the air, sinks. To keep it from sinking, then, you have to create a strong upward air current. Any bonfire will do that. If, when a cloud of gas approaches or when men carrying gas reservoirs approach the trenches, you can start a bonfire every few yards along the line, the poison gas will be sucked into this up-draught and dispersed by the heat. That has been done, several times, and it was the only defense of the British at Ypres, before the gas masks were hastily improvised. But that means hauling a lot of fuel to the front, and every pound of fuel transported means a pound less of provisions and munitions. Besides, as soon as we worked out that kind of defense, the Germans schemed a new way to use the gas. Now they put it into shells by compressed air. They have two of these gas shells which they call the 'T' type and the 'K' type." "How do we know what they call them?"