Theory of Heat

THE MERCURIAL THERMOMETER.

This thermometer consists of a glass tube ending in a bulb; the bulb and part of the tube are filled with mercury, and the rest of the tube is a vacuum original: "empty". We shall assume the tube is graduated marked with a scale of degrees in some way so that the height of the mercury in the tube may be observed and recorded. We shall not, however, assume either that the tube has a perfectly uniform diameter original: "uniform section" or that the degrees are of equal size. Therefore, the scale of this primitive thermometer must be regarded as completely arbitrary. By using our thermometer, we can determine whether one temperature is higher, lower, or equal to another, but we cannot yet claim that the difference between two temperatures, A and B, is greater or less than the difference between two other temperatures, C and D.

We shall assume that during every observation, the temperature of the mercury and the glass is equal and uniform throughout the entire thermometer. The reading on the scale will then depend on the temperature of the thermometer itself. Since we have not yet established a more perfect thermometric scale, we shall call this reading provisionally "the temperature according to the arbitrary scale of the thermometer."

The reading of a thermometer primarily indicates its own temperature. However, if we bring the thermometer into close contact with another substance—for example, if we plunge it into a liquid for a sufficient amount of time—we find that the reading of the thermometer rises or falls depending on whether the liquid is hotter or colder than the thermometer. If we leave the thermometer in contact with the substance long enough, the reading eventually becomes stationary. Let us call this final reading "the temperature of the substance." We shall find as we proceed that we are justified in doing so.

Let us now take a vessel of water which we shall assume to be at the temperature of the surrounding air, so that if left to itself it