gas

Presently he stopped and picked out a square piece of board, such as is usually left to enable plumbers to get at the joints of the gas-pipes.

“A balloon,” said Oz, “is made of silk, which is coated with glue to keep the gas in it.

But in all this country there is no gas to fill the balloon with, to make it float.”

Hot air isn’t as good as gas, for if the air should get cold the balloon would come down in the desert, and we should be lost.”