conversion of heat into work in a heat engine, a process which is independent of the nature of the medium used. A temperature scale founded on this conversion is therefore not connected with any physical property of matter, and furnishes a standard of reference to which all practical appliances for measuring temperatures may be compared.[1] When readings are expressed in terms of this scale, it is customary to use the letter K in conjunction with the number: thus 850° K would mean 850 degrees on the thermodynamic scale. When existing instruments are compared with this standard, it is found that a scale based on the assumption that the volume of a gas free to expand, or the pressure of a confined gas, increases directly as the temperature is in close agreement with the thermodynamic scale. It may be proved that if the gas employed were “perfect,” a scale in exact conformity with the standard described would be secured; and gases which approach nearest in properties to a perfect gas, such as hydrogen, nitrogen, and air, may therefore be used to produce a practical standard, the indications of which are nearly identical with the thermodynamic scale. If any other physical change be chosen, such as the expansion of a solid, or the increase in resistance of a metal, and a temperature scale be based on the supposition that the change in question varies directly as the temperature, the results obtained would differ considerably from the absolute standard. For this reason the practical standard of temperature now universally adopted is an instrument based on the properties of a suitable gas.