Question

the Mercury not fall or rise in a clinical thermometer when taken out from the mouth

Answer 1

The thermometer is not full of mercury and air, it only has mercury. The air was excluded from inside of the thermometer during the manufacturing process. Where mercury isn’t, there is a near perfect vacuum. This means there is no gas pressure above the mercury to push the mercury back down and no gas pressure keeping it from flowing down into the bulb. Liquids in an evacuated glass tube act much differently that they do with air in them. Seal a larger diameter tube with some mercury in a vacuum and tip the tube. The mercury will run down the tube and hit the sealed end with such force that the glass will break like you hit it with a hammer. There is no air to cushion the mercury at the end. If it does not break, it feels like …. well someone hit the end with a hammer. It’s a very odd feeling.Mercury and other liquids have a property called surface tension. This is an atomic level attraction between atoms and molecules. Mercury’s surface tension is very high. A drop on a table will contract into a tiny sphere with a flat bottom caused by gravity acting against this force.

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Answer 2

the murcury expands only when heat up so when it comes out from the mouth it stop rising and it does not fall because thermometer  has a kink in it to prevent   mercury from rereaching back as fast as it comes from a mouth. this is done to take a accurate measurnment of body temprature