Because there is so much alcohol, the chamber is saturated with alcohol vapor
(the gaseous form of alcohol). The dry ice keeps the bottom very cold, while
the top is at room temperature.
The high temperature at the top means that the alcohol in the felt produces
much vapor, which falls downwards.
The low temperature at the bottom means that once the vapor has fallen, it is
supercooled. That is, it is vapor
form, but at a temperature at which vapor normally can't exist. It as if you
had made steam at 95 C.
Since the vapor is at a temperature where it normally can't exist, it will very
easily condense into liquid form.
When an electrically charged cosmic ray comes along, it ionizes the
vapor--that is, tears away the electrons in
some of the gas atoms along its path. This leaves these atoms positively
charged (since it removed electrons,
which have negative charge). Other, nearby atoms are attracted to this ionized
atom. This is enough to start the
condensation process. So you see little droplets forming along the path the
particle took through the chamber.