General Science | Page 4

judges of heat. Ice water seems comparatively warm after eating ice cream, and yet we know that ice water is by no means warm. A room may seem warm to a person who has been walking in the cold air, while it may feel decidedly cold to some one who has come from a warmer room. If the hand is cold, lukewarm water feels hot, but if the hand has been in very hot water and is then transferred to lukewarm water, the latter will seem cold. We see that the sensation or feeling of warmth is not an accurate guide to the temperature of a substance; and yet until 1592, one hundred years after the discovery of America, people relied solely upon their sensations for the measurement of temperature. Very hot substances cannot be touched without injury, and hence inconvenience as well as the necessity for accuracy led to the invention of the thermometer, an instrument whose operation depends upon the fact that most substances expand when heated and contract when cooled.
[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��. 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��. 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�� and 0��, and the interval between the points is divided into 100 equal parts instead of into 180.
The boiling point of water is 212�� F. or 100�� C.
The melting point of ice is 32�� F. or 0�� 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�� 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.
Commercially, temperature readings are extremely important. In sugar refineries the temperature of the heated liquids is observed most carefully, since a difference in temperature, however slight, affects not only the general appearance of sugars and sirups, but the quality as well. The many varieties of steel likewise show the influence which heat may have on the nature of a substance. By observation and tedious experimentation it has been found that if hardened steel is heated to about 450�� F. and quickly cooled, it gives the fine cutting edge of razors; if it is heated to about 500�� F. and then cooled, the metal is much coarser and is suitable for shears and farm implements; while if it is heated but 50�� F. higher, that is, to 550�� F., it gives the fine elastic steel of watch springs.
[Illustration: FIG. 11.--A well-made commercial thermometer.]
A thermometer could be put to good use in every kitchen; the inexperienced housekeeper who cannot judge of the "heat" of the oven would be saved bad bread, etc., if the thermometer were a part of her equipment. The thermometer can also be used in detecting adulterants. Butter should melt at 94�� F.; if it does not, you may be