Why was mercury used in clinical thermometers, and why is it no longer used for that purpose?

Answer 1

Mercury was used because it has a high coefficient of expansion for a liquid, so you could heat it and it would expand, and you could use that to tell temperature. There are simpler quicker ways now (disposable plastic covers on electrical thermometers). They don't need to be washed, are quick and simple to use, not as fragile, no risk of toxic leakage. Much cheaper and easier overall.

here's a wiki-link to the old fashioned thermometer

Answer 2

In addition to having a high thermal expansion coefficient, it is extremely dense, so resetting the thermometer with a shake is easy. It is also shiny and, therefore, easy to see.

It's not used anymore because it is highly toxic and very difficult to clean up in a spill.