heat to convert any quantity of water into steam as is required to raise the same quantity of water from the freezing to the boiling point. This may be verified by mixing 1 lb. of steam at 212° with 5 lbs. of water at 32°. The latent heat becomes sensible by the condensation of the vapor, and there results 6 lbs. of water at 212°.

Examples of Cold produced by Heat becoming Latent.

232. If a few drops of ether be poured upon the hand and allowed to evaporate, a sensation of cold will be felt. The ether in evaporating extracts the heat from the hand, which becomes latent.

Damp linen feels cold when applied to the body, because the moisture in passing to a state of vapor extracts the animal heat, which entering the vapor, becomes latent.

The warm wind of summer is refreshing, because it causes a more rapid evaporation of the perspiration, which abstracts animal heat from the body to become latent in the vapor thus produced. The coolness that results from sprinkling the floor of an apartment in summer, arises from the passage of heat from a sensible to a latent state, in consequence of the evaporation of the water. For the like reason, a shower of rain is generally followed by a diminished temperature.

Water may be cooled by putting it in porous vessels. A small quantity escapes through the pores, and in evaporating abstracts a portion of heat from the remaining liquid, thus reducing its temperature. This is the process of cooling water employed in many tropical countries.

Congelation of Water and Mercury in a Vacuum.

233. When evaporation is rapidly increased, the absorption of heat is proportionally increased, and as it is taken from the surrounding objects, these are sometimes frozen. It has been stated that water may be frozen under

How is this shown? (232) Why does ether produce cold by evaporation? Why does damp linen feel cold? Why is warm wind refreshing in summer? Effect of sprinkling? Of a shower? How is water cooled in porous vessels ? (233.) Why does evaporation produce cold in surrounding objects?

the receiver of the air-pump by absorbing the vapor as rapidly as it is generated.

By operating with a liquid more volatile than water, a greater degree of cold is produced. By using sulphurous acid, which boils at 14° F., a sufficient degree of cold is produced to freeze mercury. This is effected by surrounding a thermometer bulb with cotton, saturated with sulphurous acid, and then placing it under a receiver and exhausting the air.

The rapid vaporization abstracts so much heat from the mercury that it freezes in a few minutes. If we break the bulb, the mercury is found in a solid mass like a leaden bullet. In this form mercury can be drawn out into sheets, or stamped like a coin, but it soon absorbs heat from neighboring bodies, and again passes to a liquid state

VIII. CONDENSATION OF GASES AND VAPORS.-SPECIFIC HEAT.

Causes of Condensation.

234. The CONDENSATION of a vapor, is its change from a vaporous to a liquid state. This change of state may arise from chemical action, pressure, or diminution of temper

ature.

1. Chemical action. The affinity of certain substances for the vapor of water is so strong that they absorb it from the air, even when the latter is not saturated; such, for example, are quick-lime, potash, sulphuric acid, and many others. When placed in a closed space, they in a short time abstract all of the moisture in it.

2. Pressure. If a closed cylinder is filled with vapor, and

Explain the experiment with sulphurous acid. Can mercury be frozen? (234) What is condensation of a vapor? Causes? Effect of chemical action? Examples. Effect of pressure?

this be compressed by a piston, as soon as the space occupied by the vapor is saturated, it will begin to condense, and if the pressure be continued, all the vapor will be reduced to the liquid state. Until the space becomes saturated, the pressure must be continually increased, on account of the augmented tension of the vapor, but after liquefaction begins, no further augmentation of tension takes place, and the pressure required to complete the liquefaction remains uniform.

3. Diminution of temperature.-When the temperature of any space is diminished, the amount of vapor required for saturation is diminished. After the point of saturation is reached, any further diminution of temperature causes a deposit of the vapor in a liquid form.

Steam is colorless, but when allowed to escape into the cold air, condensation takes place in the form of drops, which become visible. For the same reason, the moisture contained in the breath becomes visible in cold weather.

In winter the glass of our windows often becomes coated with drops like dew. This arises from the fact that the glass is colder than the air of the room, and thus acts continually to produce condensation of the vapor in the air. If the difference of temperature is sufficient, the particles of vapor are frozen as they are deposited, producing beautiful crystallizations. When the external air is warmer than that within, the deposit takes place on the outside of the glass. If a vessel of cold water be placed in a warm room, a deposition of moisture takes place on its exterior surface.

The nearer the air is to saturation, the more abundant is the deposit of dew. Hence, before a rain, the deposit is especially abundant. Stone walls, and the like, being cooler than the atmosphere, are often in summer covered with moisture, when they are said. to sweat. The moisture in this case is condensed from the air,

Illustrate. How long must the pressure augment? Effect of diminution of temperature? What is the color of steam? Why does it become visible? Explain the deposition of drops on glass. Explain frost-work crystals. Why is the deposition more abundant before rain? Deposition on stones and walls?

and does not come from the stones. If the sweating of stones is indicative of rain, it is because the deposition is most abundant when the air is most nearly saturated.

Heat developed by Condensation.

235. When a liquid passes to a state of vapor, a great quantity of heat is absorbed from neighboring bodies, and becomes latent. When the vapor returns to a liquid state, an equal amount of heat is given out and becomes capable of affecting our senses; in other words, it becomes sensible.

Heating by Steam.

236. Buildings are heated by means of steam conveyed from a boiler in the lower story, through iron pipes in the walls. The steam, by its heat and by the heat given out on condensation, serves to warm the apartments through which it is made to pass. To this end, coils of pipes are placed in the rooms to be warmed.

Distillation.

237. DISTILLATION is the process of separating liquids from each other by means of heat.

The most volatile of the liquids is most easily evaporated, and its vapor is then condensed. The heat should be kept above the boiling point of the liquid that we wish to obtain, but below that which we wish to leave behind. The boiling point of alcohol being 174° F., and that of water 212°, if a mixture of alcohol and water be heated up to some temperature between these limits, the alcohol will all be vaporized, whilst most of the water will remain behind.

Why indicative of rain? (235.) Explain the development of heat by condensation? (236.) Explain the principle of heating buildings by steam? (237.) What is distillation? What degree of heat is required for distillation?

3

The Alembic.

238. An ALEMBIC, or Still, is an apparatus for distillation.

The most usual form of an alembic is represented in Fig. 152. It is composed of a boiler, A, with a cover, B,

called the dome; from the top of the dome a metallic tube, C, passes into a vessel, S, called the condenser, and is then bent into a spiral form. This tube is called the worm, and after passing through the condenser, S, it leads to a receiver, D. The condenser, S, is kept full of cold water by an arrangement shown in the figure.

The substance to be distilled is placed in A, and a suitable heat is then applied. The more volatile portion is converted