Why is mercury now seldom seen in laboratories?
Are you asking why we don't see mercury in barometers anymore?
Safety considerations are the obvious reasons. Mercury is (i) extremely dense (13.55
As a student myself, I have floated pound coins in it, used it for electrical switches, and later (legitimately!) dissolved alkali metals in it for use in reductions. Should your container break (inevitable even with grad students), however, you've got droplets of mercury everywhere; and the metal will inhabit every pinhole and penetrate every crack. This is a major cleanup job, which contract cleaners won't touch. There are now also some good alternatives to the use of mercury for reduction.
Instruments that measure pressure by electronic means have also meant the gradual phasing out of the old mercury manometer.
I think you are asking how the mercury works in the barometer.
A mercury barometer consists of a reservoir of mercury that is exposed to the atmosphere. In this reservoir sits a vacuum tube. The weight of the atmosphere on the reservoir will force the mercury up into the vacuum tube where there is no weight. This is air pressure. The standard atmospheric pressure is enough to push the mercury 29.92 inches up the vacuum tube.
The reason we use mercury is that it allows us to be more accurate. If we were to use water, and had a tube the same diameter it would need to be over 20 ft tall (I recall hearing an exact number during my training but I cannot remember it). If we were to use a tube of much greater diameter we would lose accuracy.
Anor is quite right about the hazards of mercury. The sale of mercury has been greatly restricted of recent and no one is making new mercury barometers that I am aware of. We use accurate digital barometers now that have all, oddly enough, been calibrated using an accurate and reliable mercury barometer.