"Which were the gases used at Ypres, where the poison gas business first began?"
"Chlorine and bromine," the other answered, "so this chemist chap told me. They get the chlorine by passing strong currents of electricity through sea-water by some process he explained but which I couldn't understand; and the bromine is a by-product that they make from the Strasburg salts. But there's some other gases like sulphurous anhydride and carbonyl chloride that I don't know much about."
"Did you find out how it is that the masks really prevent poisoning?" the boy asked.
"That's simple enough. Chlorine and bromine have what this chemist fellow called an 'affinity' for alkalies, and the gas combines with the alkali somehow, so that all the poisonous effect is lost. French, English and German masks are different in shape, but the idea is the same. The Germans have a mask which fits over the nose and mouth, filled with absorbent cotton treated with hyposulphite of sodium or sodium carbonate. The French and English have a mask that covers the whole head and which can be tucked under the collar of the tunic.
"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."