[Nottingham] Mount and Filesystem

[NLUG]

Hey guys,

I have a couple of questions for you.
Let's say you have a 30GB partition /var on disk 1 (/dev/sda1).
We also have a 100GB disk 2 which is one big /dev/sdb1 partition.
The 30GB /var is nearly full...if i mount /dev/sdb1 to /var, what happens?

Do i get a 130GB partition?
if an application keeps adding stuff to /var, where is the data written 
to ? does it fill up /dev/sda1 first and then carries on to /dev/sdb1?
Or more simply, can you not mount a partition onto an already mounted 
partition ?

i'm intrigued :-)
Thanks in advance.

Raph