The temperature is not a measure of the quantity of heat a body possesses. Temperature may be considered to be a measure of the velocity with which the molecules of a body vibrate to and fro, while the quantity of heat may be considered to be the total energy of the molecules composing the body. A small iron rod may be heated to whiteness and yet possess a very small quantity of heat. Its temperature is very high, but this simply indicates that the molecules of the rod are vibrating with an extremely high velocity. An iron ball 1 foot in diameter and an iron ball 1 inch in diameter may have exactly the same temperature, but the larger ball will have by far the greater quantity of heat.
5. Sensible Heat. — The simplest way by which heat is recognized is by the sense of touch. If a particular object contains a large amount of heat, it feels hot; and between two objects made of the same material,, the sense of touch is an approximate guide to their relative temperatures. On the other hand, between two objects made of different materials, as, for example, iron and cloth, the sense of touch may reveal nothing at all concerning their relative temperatures. The reason is that the sense of touch indicates not the temperature directly, but rather the rapidity with which heat is transferred from the object to the hand, or is abstracted from the hand if the object is colder than the hand. The heat that thus manifests itself is called sensible neat, because any change of the same body from any state to a hotter or colder state is indicated at once by the sense of feeling, or with the aid of instruments called thermometers. The more sensible heat a body possesses, the hotter it is; the more sensible heat that is taken away from it, the colder it is.
6. Measurement of Temperature. — As just stated, temperature is measured by means of an instrument called a thermometer, one type of which is shown in Fig. 1. It consists of a glass tube, closed at both ends and having a bulb at the lower end. The bulb and the lower end of the tube are filled with mercury, which, on being heated or cooled, expands or contracts in proportion to the change of
