# Why $Q_{f}$ on glass ~ $12Q_f$ on air in Leyden Jar Expt?

I watching a video lecture on the internet and at at about 46:55 the professor says that the free charge ($Q_f$) on the glass is about 12 times that of air in an experiment with a Leyden Jar. He says it is a straightforward calculation, but I don't see it yet.

Many thanks

http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/lecture-14-biot-savart-law/

-