General Science | Page 5

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 sure that it is
adulterated with suet or other cheap fat. Olive oil should be a clear
liquid above 75° 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, provided with a
definite inlet for air and a definite outlet for smoke, and able to radiate
into the room most of the heat produced from the fire which burns
within. The inlet, or draft, admits enough air to cause the fire to burn
brightly or slowly as the case may be. If we wish a hot fire, the draft is
opened wide and enough air enters to produce a strong glow. If we
wish a low fire, the inlet is only partially opened, and just enough air
enters to keep the fuel smoldering.
When the fire is started, the damper should be opened wide in order to
allow the escape of smoke; but after the fire is well started there is less
smoke, and the damper may be partly closed. If the damper is kept open,
coal is rapidly consumed, and the additional heat passes out through the
chimney, and is lost to use.
9. Furnaces. Hot Air. The labor involved in the care of numerous stoves
is considerable, and hence the advent of a central heating stove, or
furnace, was a great saving in strength and fuel. A furnace is a stove
arranged as in Figure 13. The stove S, like all other stoves, has an inlet
for air and an outlet C for smoke; but in addition, it has built around it a
chamber in which air circulates and is warmed. The air warmed by the
stove is forced upward by cold air which enters from outside. For
example, cold air constantly entering at E drives the air heated by S
through pipes and ducts to the rooms to be heated.
The metal pipes which convey the heated air from the furnace to the
ducts are sometimes covered with felt, asbestos, or other
non-conducting material in order that heat may not be lost during
transmission. The ducts which receive the heated air from the pipes are
built in the non-conducting walls of the house, and hence lose