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. 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 deg. F. and quickly cooled, it gives the fine cutting edge of razors; if it is heated to about 500 deg. F. and then cooled, the metal is much coarser and is suitable for shears and farm implements; while if it is heated but 50 deg. F. higher, that is, to 550 deg. 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 deg. F.; if it does not, you may be sure that it is adulterated with suet or other cheap fat. Olive oil should be a clear liquid above 75 deg. F.; if, above this temperature, it looks cloudy, you may be sure that it too is adulterated with fat. 8. Methods of Heating Buildings. _Open Fireplaces and Stoves._ Before the time of stoves and furnaces, man heated his modest dwelling by open fires alone. The burning logs gave warmth to the cabin and served as a primitive cooking agent; and the smoke which usually accompanies burning bodies was carried away by means of the chimney. But in an open fireplace much heat escapes with the smoke and is lost, and only a small portion streams into the room and gives warmth.