[Illustration:  Fig. 8.—­Making a thermometer.]

6.  The Thermometer.  The modern thermometer consists of a glass tube at the lower end of which is a bulb filled with mercury or colored alcohol (Fig. 8).  After the bulb has been filled with the mercury, it is placed in a beaker of water and the water is heated by a Bunsen burner.  As the water becomes warmer and warmer the level of the mercury in the tube steadily rises until the water boils, when the level remains stationary (Fig. 9).  A scratch is made on the tube to indicate the point to which the mercury rises when the bulb is placed in boiling water, and this point is marked 212 deg..  The tube is then removed from the boiling water, and after cooling for a few minutes, it is placed in a vessel containing finely chopped ice (Fig. 10).  The mercury column falls rapidly, but finally remains stationary, and at this level another scratch is made on the tube and the point is marked 32 deg..  The space between these two points, which represent the temperatures of boiling water and of melting ice, is divided into 180 equal parts called degrees.  The thermometer in use in the United States is marked in this way and is called the Fahrenheit thermometer after its designer.  Before the degrees are etched on the thermometer the open end of the tube is sealed.

[Illustration:  Fig. 9.—­Determining one of the fixed points of a thermometer.]

The Centigrade thermometer, in use in foreign countries and in all scientific work, is similar to the Fahrenheit except that the fixed points are marked 100 deg. and 0 deg., and the interval between the points is divided into 100 equal parts instead of into 180.

The boiling point of water is 212 deg.  F. or 100 deg.  C.

The melting point of ice is 32 deg.  F. or 0 deg.  C.

Glass thermometers of the above type are the ones most generally used, but there are many different types for special purposes.

[Illustration:  Fig. 10.—­Determining the lower fixed point of a thermometer.]

7.  Some Uses of a Thermometer.  One of the chief values of a thermometer is the service it has rendered to medicine.  If a thermometer is held for a few minutes under the tongue of a normal, healthy person, the mercury will rise to about 98.4 deg.  F. If the temperature of the body registers several degrees above or below this point, a physician should be consulted immediately.  The temperature of the body is a trustworthy indicator of general physical condition; hence in all hospitals the temperature of patients is carefully taken at stated intervals.