-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.

But open fireplaces, while pleasant to look at, are not efficient for
either heating or cooking. The possibilities for the latter are
especially limited, and the invention of stoves was a great advance in
efficiency, economy, and comfort. A stove is a receptacle for fire,
provi