Soap-bubbles
Giant soap bubbles made tough with a special glue.
First named in The Road to Oz, ch. 24
He had invented a machine to blow huge soap-bubbles, as big as balloons
As the books describe them
Usually, soap-bubbles are frail and burst easily, lasting only a few moments as they float in the air; but the Wizard added a sort of glue to his soapsuds, which made his bubbles tough; and, as the glue dried rapidly when exposed to the air, the Wizard's bubbles were strong enough to float for hours without breaking. The Road to Oz, ch. 24