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.

When fuel is placed in an open fireplace (Fig. 12) and lighted, the air immediately surrounding the fire becomes warmer and, because of expansion, becomes lighter than the cold air above.  The cold air, being heavier, falls and forces the warmer air upward, and along with the warm air goes the disagreeable smoke.  The fall of the colder and heavier air, and the rise of the warmer and hence lighter air, is similar to the exchange which takes place when water is poured on oil; the water, being heavier than oil, sinks to the bottom and forces the oil to the surface.  The warmer air which escapes up the chimney carries with it the disagreeable smoke, and when all the smoke is got rid of in this way, the chimney is said to draw well.

[Illustration:  Fig. 12.—­The open fireplace as an early method of heating.]

As the air is heated by the fire it expands, and is pushed up the chimney by the cold air which is constantly entering through loose windows and doors.  Open fireplaces are very healthful because the air which is driven out is impure, while the air which rushes in is fresh and brings oxygen to the human being.