Because mercury is harmful for us, it has been replaced by alcohol in a thermometer.
[Notes: (i) Mercury thermometers are still widely used in laboratories in schools and colleges.
(ii) A thermometer is a device to measure temperature. A thermometer containing mercury in its bulb is called a mercury thermometer. There are other types of thermometer such as a thermocouple thermometer, a platinum resistance thermometer, a thermistor thermometer, etc.]