From the video description:
This video shows the Cloudylabs's cloud chamber running for approx. 50 min with an Uranium mineral. After 40 min, there is not enough alcohol to make newer trails. With time, the alcohol condense on the mineral. The small thickness of liquid alcohol on the mineral is enough to absorb a part of the energy of the alpha particles (their ranges in air for 5 MeV is 3-4 cm, but in water, it's 15 micrometer), so with time, the trails are shorter than in beginning. It's preferable to make such experience during 10 minutes to have longer alpha track.