What is the earliest reference to a bubble floating in the air?
And I often, with a little tube, Made a soap bubble fly from the water.
I'm not sure when he wrote this but he died in about 1405.
Also, there's a picture of a monkey maybe blowing a bubble.
Detail from a full border of a monkey playing a game (possibly blowing bubbles?), from the Isabella Breviary, Southern Netherlands (Bruges), late 1480s and before 1497, British Library, Additional 18851, f. 470v
OK, it's not quite in the air but it's close. There are some better ones at http://willscommonplacebook.blogspot.com/2013/02/blowing-bubbles.html but they aren't as old. Here's one example from around 1617 in a picture called Homo Bulla by Karel van Sichem.