“Sure,” Donald grinned. “Air contains six gases. The principal ones are oxygen and nitrogen. Oxygen is used a great deal, nitrogen very little, except in time of war.
“But the other gases are used a great deal too. Ever walk down the streets of a big city at night and notice all the gleaming, flashing signs?” he asked.
“Sure have!”
“Remember the inches of glass tubing all full of something that glowed red, blue, green, yellow?”
“Sure.”
“Well, those tubes each contain gas, krypton, argon, or neon. That’s why they are called neon signs. A great deal of that gas comes from liquid air or is separated by the aid of it. So you see, if we can supply manufacturers with clean, cheap liquid air we have—”
“A fortune!” Johnny drew in a long breath. “How wonderful!”
“Well,” Donald said slowly, “perhaps not a fortune but a chance to live and to help others a little, and that is something these days.
“Liquid air,” he went on after a moment, “makes a wonderful explosive. You see the oxygen in liquid air is free to join with carbon. All you have to do is to soak charcoal in liquid air, attach a fuse to it, scratch a match then run. The result is a glorious explosion.”
“Swell for Fourth of July!” Johnny enthused.