I assume you are referring to
, how they did it in the film was explained in the plot, but in real life would be performed by running behind the scenery at the back of the stage e.g.
.

The water tank is rigged for easy escape as houdini did it.
Telportaion was always done with a double just as modern day lamo magicians do.

https://www.google.co.uk/search?q=pepper's+ghost+ninth+century&tbm=isch

yes but how can he do this in 19 century?

No, it is not a mirror, it is a piece of glass at 45°. If you google 'peppers ghost' it will show you or search youtube to see videos like

is this really possible with the mirror?

All of the tricks are based on real tricks but are not realistic, for example
is based on 'Houdin's orange tree' illusion shown here by Paul Daniels
and the ghost trick
is based on 'Pepper's ghost' eg