It was the type where the hydrogen obeys the laws P1V1/T1 = P2V2/T2. The balloons did leak a bit and hydrogen does behave slightly oddly but the equation is a good enough analogy to show what happens when P reduces by a lot and T reduces by a little (in terms of Kelvin).
If you added a bit too much gas then the balloon would burst at a lower altitude so you could bunk off home sooner. Not that I ever did such a thing
