Does smoke rise or goes down?

To realize our experiment we need a bottle of plastic, a sheet of paper and a few matches.

First we do a couple of holes in the bottle of plastic, one in the top part and another one near the base of the bottle.
Then we take the sheet paper and cut away a rectangle of 10x15 cm.
We coil the paper to obtain a small cylinder of approximately 15 cm of length.
Finally the small tube of paper interferes for the top hole of the bottle.

On having ignited the small tube of paper with a match a small flame is formed and is observed that for another end of the bead there goes out a column of very dense smoke that falls down inside the bottle. In the exterior scarcely there is smoke.

If we cover the low hole with a finger the small tube of paper goes out and smoke does not go out.

Explanation:

On having burned the small tube it departs from the paper it is clear in the shape of particles that, together with the gases that are formed in the combustion and the air they form the smoke.
In normal circumstances, the smoke ascends dragged by the warm air of the combustion (currents of convection).

In our experiment, the smoke that takes place is produced in the interior part of the small tube travels along it.
Inside the bottle there is no warm air, so that when the smoke goes out for the low end of the small tube there do not take place (there are not produced rising currents of convection) and the smoke (denser than the air) it rushes to the bottom of the bottle.

by Francisco and Fernando