How to reference a Particle System?

I normally have no problems accessing other scripts and such, but this is my first time using a particle system. I would THINK that it would be:

 PartSystem = gameObject.GetComponent(ParticleSystem);
 PartSystem.emit = true;

But this doesn't work. I am trying to make the system emit while a variable is true, and stop while it is not.

Edit: it gives me the error "Object not set to an instance of an object"