When the air of a room becomes warmed and vitiated by the presence of a number of persons, it expands and becomes lighter than the external air; hence it rises to the top of the room, and its place is supplied by fresh air from without, which enters through the cracks of the doors, or through apertures constructed for the purpose. Openings should be made at the upper part of the room to permit the foul air to escape. Such is the theory of ventilation of rooms.

In large buildings, like theatres, the spectators in the upper galleries often experience great inconvenience from the hot and corrupt air arising from below. To remedy this evil, large openings, called ventilators, should be constructed in the ceiling, and corresponding openings should be arranged near the bottom of the building, to supply a sufficient quantity of fresh air to keep up the circulation.

The principal of expansion gives a draft to our chimneys. The hot air ascends through the flue, and its place is supplied by a continued current of cold air from below, which keeps up the combustion in the fire-place or grate.

The same principle is applied in warming buildings by means of furnaces. Furnaces are placed in the lowest story of the building, and are provided with air chambers, which communicate with the external air by means of air-pipes. When the air becomes heated in the air chamber, it rises through pipes, or flues in the walls, to the upper stories of the building, and is admitted to or excluded from the different apartments by valves, called registers.

The principle of expansion of air explains many meteorological phenomena. When the air in any locality becomes heated by the rays of the sun, it rises and its place is supplied by colder air from the neighboring regions, thus producing the phenomena of winds. The circulation of the atmosphere in the form of winds, tends to equalize the temperature, and also, by transporting clouds and vapors, tends to equalize the distribution of water over the globe.

Winds also serve to remove the vitiated air of cities, replacing it by the pure air of the neighboring places, thus contributing to the preservation of life and health. Winds also act to propel vessels on

(214.) How does the principle of expansion operate in ventilation? How are large buildings ventilated? What gives draft to chimneys? Explain the theory of heating by furnaces. How does the principle of expansion produce winds! Their effect on distribution of warmth and moisture!

the ocean, thus contributing to the spread of commerce and civilization.

Without winds, our cities would become centres of infection, the clouds would remain motionless over the localities where they were formed, the greater portion of the earth would become arid and desert, without rivers or streams to water them, and the whole earth would soon become uninhabitable.

Density of Gases.

215. The density of a gas depends upon the pressure to which it is subjected, and also upon its temperature.

It is for this reason that we select as a term of comparison the density at some particular pressure and temperature. The standard pressure is that of the atmosphere when the barometer stands at 30 inches, and the standard temperature is 32° F., or the freezing point of water. To determine the density at any other pressure, we apply MARIOTTE'S law; to determine it at any other temperature, we apply the coefficient of expansion, as explained in preceding articles.

Suppose it were required to determine the density of air when the barometer indicates 20 inches, and the thermometer 62° F., the density being equal to 1 at the standard temperature and pressure. The pressure being only two thirds the standard pressure, the air in the case considered would occupy once and a half its primitive volume, supposing the temperature to remain at 32° F. But the temperature being 620 F., or 30° above the standard, we multiply 1.5 by 30 times 0.00204 for the expansion. This product, added to 1.5, gives for a result, 1.5918. That is, a unit of volume at the standard pressure and temperature becomes 1.5918 units of volume at the given pressure and temperature. Because the density varies inversely as the volume, we shall have for the required density, T., or 0.6282.

Other effects of winds? (215.) On what does the density of a gas depend? What do we take as a standard? How do we determine the density at any other pressure and temperature? Example,

The following table exhibits the density of some of the most important gases, air being taken as a standard:

Hydrogen is the lightest known body, its density being fourteen and a half times less than that of air.

VI.-CHANGE OF STATE OF BODIES BY THE ACTION OF HEAT.

Fusion.

216. It has been stated that heat not only causes bodies to expand, but that it may in certain circumstances cause them to change from the solid to the liquid state, or from the liquid to the gaseous state.

When a body passes from a solid to a liquid state, it is said to melt, or fuse, and the act of changing state in this case is called fusion.

If a melted body is suffered to cool, it becomes solid at the same temperature at which it melted. Hence the melting point is the same as the freezing point.

Fusion takes place when the force of cohesion, which holds the particles of a body together, is exactly balanced by the heat which tends to separate them. The temperature at which fusion takes place is different for different bodies. For some bodies it is very low, and for others very high, as is shown in the following

What is the lightest body? Give the densities of some other gases. (216) What is melting or fusion? When does fusion take place? Is the melting point the same for all solids 1

Some are de

Simple bodies,

All bodies are not melted by the action of heat. composed, such as paper, wood, bone, marble, &c. that is, bodies which are composed of but one kind of matter, always melt, if sufficiently heated, with a single exception. Carbon has thus far resisted all attempts to fuse it.

Latent Heat of Fusion.

217. Bodies which can be melted always present the remarkable phenomenon, that when they are heated to the temperature of fusion, they can not be heated any higher until the fusion is complete. For example, if ice be exposed to heat, it begins to melt at 32° F., and if more heat be applied, the melting is accelerated, but the temperature of the mixture of ice and water remains at 32° until all the ice is melted.

The heat that is applied during the process of fusion, enters into the body without raising its temperature, and is said to become latent. When the body returns to its solid state, all the latent heat is again given out, and once more becomes sensible.

The phenomenon of latent heat may be illustrated by the following experiment. If a pound of pulverized ice, at 32° F., be mixed

Examples. Are all bodies melted by the action of heat? Examples. (217) What is latent heat! Sensible heat? How may the phenomenon of latent heat be illustrated.

with a pound of water at 174° F., the heat of the water will be just sufficient to melt the ice, and there will result two pounds of water at the temperature of 32° F. During the process of melting, 142° of heat have been absorbed and become latent; hence, we say that the heat required to melt ice at 32° F. is 142°, or, in other words, the latent heat of water at 32° is 142°.

The enormous amount of heat which becomes latent when ice melts, explains why it is that large masses of ice remain unmelted for a considerable time after the temperature of the air is raised above 32° F. Conversely, the immense quantity of heat evolved when water passes to the state of ice, explains why it is that ice forms so slowly in extremely cold weather. The absorption of heat in melting, and production of heat in freezing, tend to equalize the temperature of climates in the neighborhood of large masses of water, like lakes and rivers.

Congelation.-Solidification.

218. Any body that can be melted by the application of heat, can be brought back to a solid state by the abstraction of heat. This passage from a liquid to a solid state is called congelation, or solidification.

In every body, the temperature at which congelation commences, is the same as that at which fusion begins. Thus, if water be cooled, it will begin to congeal at 32° F., and conversely, if ice be heated, it will begin to melt at 32° F. Furthermore, the amount of heat given out, or rendered sensible in congealing, is exactly equal to that absorbed, or rendered latent in melting.

Some liquids can not be congealed by the greatest cold to which we can subject them; such are alcohol and ether. Pure water congeals at 32°; the salt water of the ocean congeals at 27°; olive oil at 21° linseed and nut oils at 17°.

Explain the action of latent heat on melting masses of ice. Also on freezing masses of water. (218.) What is congelation? How does the point of congelation compare with that of fusion? Illustrate. How does the heat given out in solidifying compare with that taken up in melting? What liquids have never been frozen ?