[Chaste]

I think it's also called the Nujol bubbler. As above 

[dunno260]

Assuming what you are talking about is hooked up to an inert gas line (or gas line of some sort) its to keep an inert atmosphere in the flask and act as a one way valve of air flow.  Your inert gas can flow through and escape so there isn't a pressure buildup, but air can't get back in (assuming you don't build up a vacuum). 

[Chaste]

oh.. so the oil is specific in its permissivity? it only allows gaseous nitrogen to escape but forbid other gas to enter it?

[superg]

usually nitrogen is always bubling, to make sure there is no air entering in the system (schleck line with blassware attached to it with rubber tubbing most of the time).

but if you create an underpressure in your system (by applying vacuum or filing with nitrogen glassware previously under vacuum), the nitrogen stop bubling as it is use to fill the vaccuum. also if there is a leak in your system, it stops bubling. but if you don't take any precaution, you can suck all the oil and air can enter.

the bubler is not compulsory, but it is a safety to check if the system is under inert gas or not.

[jake.n]

You can also accomplish the same thing by needling in a nitrogen line through a septum.  Keep positive pressure on the nitrogen line and leave another needle in the septum to prevent buildup of pressure in your flask.

[Chaste]

thanks guys. I read it off a source that it keeps the apparatus inert by allowing nitrogen to be bubbled through and prevents external air from entering the set up.