A thermometer is described which detects temperature changes of the order of 10−4°C and quantities of heat of the order of 10−4g cal. It is essentially a gas or vapor thermometer in which the shift of the top of the meniscus of an indicating liquid is observed through a microscope. The shift isdue to changes of the shape of the meniscus while the circumference remains stationary.Hence the sensitivity depends upon the surface tension of the indicating liquid but not on the friction between the liquid and the glass.