Introduction.

The volume of most substances increases continuously as the temperature rises, provided the pressure remains constant. There are exceptions to this rule, and the expansions original: "dilatations" of different substances are not generally in the same proportion; however, any substance in which an increase of temperature—no matter how small—produces an increase in volume may be used to indicate changes in temperature.

For instance, mercury and glass both expand when heated, but the expansion of mercury is greater than that of glass. Hence, if a cold glass vessel is filled with cold mercury, and if the vessel and the mercury within it are then equally heated, the glass vessel will expand, but the mercury will expand more. Consequently, the vessel will no longer be able to contain all the mercury. If the vessel is equipped with a long neck, the mercury forced out of the vessel will rise into that neck; if the neck is a narrow tube with fine markings original: "graduated", the amount of mercury forced out of the vessel may be accurately measured.

This is the principle of the common mercurial thermometer, the construction of which will be described more minutely later. At present, we consider it simply as an instrument whose readings original: "indications" vary when the temperature varies, but are always the same when the temperature of the instrument is the same.

The expansion of other liquids, as well as that of solids and gases, may be used for measuring temperature original: "thermometric purposes". Additionally, the thermo-electric properties of metals and the variation of their electrical resistance with temperature are also employed in research on heat. We must first, however, study the theory of temperature itself before we examine how the properties of different substances relate to temperature. For this purpose, we shall use the specific mercurial thermometer just described.