DRAUGHT OF CHIMNEYS.

299

The draught is not so good when a fire is first lighted as after it has been burning for some time, because a cold chimney chills the air within it. On the other hand, if the fire is so regulated as to keep the room at the same temperature in all weathers, the draught will be strongest when the weather is coldest.

The opening at the lower end of the chimney should not be too wide nor too high above the fire, as the air from the room would then enter it in large quantity, without being first warmed by passing through the fire. These defects prevailed to a great extent in old chimneys. Rumford was the first to attempt rational improvements. He reduced the opening of the chimney and the depth of the fireplace, and added polished

plates inclined at an angle, which serve both to guide the air to the fire and to reflect heat into the room (Fig. 220).

The blower (Fig. 221) produces its well-known effects by compelling all air to pass through the fire before entering the chimney. This at once improves the draught of the chimney by raising the

temperature of the air within it, and quickens combustion by increasing the supply of oxygen to the fuel.

224. Stoves. The heating of rooms by open fire-places is effected almost entirely by radiation, and much even of the radiant heat is wasted. This mode of heating then, though agreeable and healthful, is far from economical. Stoves have a great advantage in point of economy, for the heat absorbed by their sides is in great measure given out to the room, whereas in an ordinary fire-place the greater part of this heat is lost. Open fire-places have, however, the advantage as regards ventilation; the large opening at the foot of the chimney, to which the air of the room has free access, causes a large body of air from the room to ascend the chimney, its place being supplied by fresh air entering through the chinks of the doors and windows, or any other openings which may exist.

Stoves are also liable to the objection of making the air of the room too dry, not, of course, by removing water, but by raising the tem

perature of the air too much above the dew-point (Chap. xxviii.). The same thing occurs with open fire-places in frosty weather, at which time the dew-point is unusually low. This evil can be remedied by placing a vessel of water on the stove. The reason why it is more liable to occur with stoves than with open fire-places, is mainly that the former raise the air in the room to a higher temperature than the latter, the defect of air-temperature being in the latter case compensated by the intensity of the direct radiation from the glowing fuel.

Fire-clay, from its low conducting power, is very serviceable both for the backs of fire-places and for the lining of stoves. In the former

As

situation it prevents the wasteful
escape of heat backwards into the
chimney, and keeps the back of the
fire nearly as hot as the centre.
a lining to stoves, it impedes the
lateral escape of heat, thus answer-
ing the double purpose of prevent-
ing the sides of the stove from over-
heating, and at the same time of
keeping up the temperature of the
fire, and thereby promoting com-
plete combustion. Its use must,
however, be confined to that por-
tion of the stove which serves as
the fire-box, as it would otherwise
prevent the heat from being given
out to the apartment.

The stove represented in Fig. 2221 belongs to the class of what are called in France calorifères, and in England ventilating stoves, being constructed with a view to promoting the circulation and renewal of the air of the apartment. G is the fire-box, over which is the feeder U, containing unburned fuel, and tightly closed at top by a lid, which is removed only when fresh fuel is to be introduced. The ash-pan F has a door pierced with holes for admitting air to support com

1 With the exception of the ventilating arrangement, this stove is identical with what is known in this country as Walker's self-feeding stove.

VENTILATING STOVE.

301 bustion. The flame and smoke issue at the edge of the fire-box, and after circulating round the chamber O which surrounds the feeder, enter the pipe T which leads to the chimney. The chamber O is surrounded by another inclosure L, through which fresh air passes, entering below at A, and escaping into the room through perforations in the upper part of the stove as indicated by the arrows. The amount of fresh air thus admitted can be regulated by the throttlevalve P.

CHAPTER XXIV.

FUSION AND SOLIDIFICATION.

225. Fusion.-Many solid bodies, when raised to a sufficiently high temperature, become liquid. This change of state is called melting or fusion, and the temperature at which it occurs (called the meltingpoint, or temperature of fusion) is constant for each substance, with the exception of the variations-which in ordinary circumstances are insignificant-due to differences of pressure (§ 237). The meltingpoints of several substances are given in the following table:

TABLE OF MELTING-POINTS, IN DEGREES CENTIGRADE.

Some bodies, such as charcoal, have hitherto resisted all attempts to reduce them to the liquid state; but this is to be attributed only to the insufficiency of the means which we are able to employ.

It is probable that, by proper variations of temperature and pressure, all simple substances, and all compound substances which would not be decomposed, could be compelled to assume the three forms, solid, liquid, and gaseous.

The passage from the solid to the liquid state is generally abrupt;

LATENT HEAT OF FUSION.

303

but this is not always the case. Glass, for instance, before reaching a state of perfect liquefaction, passes through a series of intermediate stages in which it is of a viscous consistency, and can be easily drawn out into exceedingly fine threads, or moulded into different shapes.

226. Constant Temperature during Fusion.-During the entire time of fusion the temperature remains constant. Thus if a vessel containing ice be placed on the fire, the ice will melt more quickly as the fire is hotter; but if the mixture of ice and water be constantly stirred, a thermometer placed in it will indicate the temperature zero without variation so long as any ice remains unmelted; it is only after all the ice has become liquid that a rise of temperature will be observed.

In the same way, if sulphur be heated in a glass vessel, the temperature indicated by a thermometer placed in the vessel will rise gradually until it reaches about 110°, when a portion of the sulphur will be seen to become liquid, and if the vessel be shaken during the time of fusion, until the whole of the sulphur is liquefied, the temperature will be observed to remain steadily at this point.

227. Latent Heat of Fusion.

This con

stancy of temperature is very remarkable,

Fig. 223.-Fusion of Sulphur.

and leads to some important conclusions. In fact, as the action of the fire continues the same throughout the entire time of fusion, while the thermometer remains stationary, all the heat supplied after liquefaction has begun, appears to be lost. Hence we conclude, that in order that a body may pass from the solid to the liquid state, it must absorb a certain quantity of heat which produces no thermometric effect. Black, who was the first to investigate this subject, gave to the heat thus absorbed the name of latent heat, by which it is still usually designated. A similar absorption of heat without thermometric effect occurs when a boiling liquid is converted into vapour. Hence it is necessary to distinguish between the latent heat of fusion and the latent heat of vaporization. Latent heat then may be defined as the heat absorbed in virtue of change of

1

1 The former is often called the latent heat of the liquid, and the latter of the vapour. Thus we speak of the latent heat of water (which becomes latent in the melting of ice), and of the latent heat of steam (which becomes latent in the vaporization of water).