CHAPTER XII.. 4.--Evaporation.

4.--EVAPORATION.

Q. What is meant by EVAPORATION?

A. The dissipation of liquid by its being converted into vapour.

Q. What EFFECTS are produced by evaporation?

A. The liquid vaporized absorbs heat from the body whence it issues; and the body deprived of the liquid by evaporation, loses heat thereby.

Q. If you WET your FINGER in your mouth, and hold it up in the air, why does it FEEL COLD?

A. The saliva quickly evaporates; and (as it evaporates) absorbs heat from the finger, which makes it feel cold.

Q. If you BATHE your TEMPLES with ether, why does it allay INFLAMMATION and feverish heat?

A. Ether very rapidly evaporates; and (as it evaporates) absorbs heat from the burning head, producing a sensation of cold.

Q. Why is ETHER better for this purpose than WATER?

A. Because it requires less heat to convert it into vapour; and therefore it evaporates much more quickly.

(Ether is converted into steam with 104 degs. of heat, but water requires 212 degs. of heat to convert it into steam.)

Q. Why does ETHER very greatly RELIEVE a SCALD or BURN?

A. Because it evaporates very rapidly; and (while it is converted into vapour) carries off the heat of the burn.

Q. Why do we FEEL so COLD when we have WET FEET or CLOTHES?

A. As the wet of our shoes or clothes evaporates, it keeps absorbing heat from the body, which makes it feel cold.

Q. Why do WET FEET or CLOTHES give us "COLD?"

A. Because the evaporation absorbs heat from the body so abundantly, that it is lowered below its natural standard; and therefore health is injured.

Q. Why is it DANGEROUS to SLEEP in a DAMP BED?

A. Because the heat of the body is continually absorbed in converting the damp of the sheets into vapour; and as heat is abstracted from the body, its temperature is reduced below the healthy standard.

Q. Why do we not feel the same sensation of cold, if we throw a MACINTOSH over our WET CLOTHES?

A. The macintosh prevents evaporation, because the steam cannot escape through the air-tight fabric; and (as the wet cannot evaporate from the clothes) no heat is absorbed from our bodies.

Q. Why do NOT SAILORS get COLD, who are so often wet all day with SEA-WATER?

A. The salt of the sea retards evaporation; and (as the heat of the body is drawn off very gradually) the sensation of cold is prevented.

Q. Why does SPRINKLING a HOT ROOM with water COOL IT?

A. The heat of the room causes a rapid evaporation of the sprinkled water; and as the water evaporates, it absorbs heat from the room, and cools it.

Q. Why does WATERING the STREETS and roads COOL THEM?

A. The hot streets and roads part with their heat to promote the evaporation of the water sprinkled on them.

Q. Why does a SHOWER of RAIN seem to COOL the AIR in summer-time?

A. The earth (being wet with the rain) parts with its heat to promote evaporation; and as the earth is cooled, it cools the air also.

Q. Why is LINEN DRIED by being exposed to the WIND?

A. The air (blowing over the linen) promotes evaporation, by removing the vapour from the surface of the wet linen, as soon as it is formed.

Q. Why is LINEN DRIED sooner in the open AIR, than in a confined room?

A. Because the particles of vapour are more rapidly removed from the surface of the linen by evaporation.

Q. Why are WET SUMMERS generally SUCCEEDED by COLD WINTERS?

A. Because the great evaporation (carried on through the wet summer) reduces the temperature of the earth lower than usual, and produces cold.

Q. Why is ENGLAND WARMER than it used to be, when AGUES were so common?

A. Because it is better drained and better cultivated.

Q. Why does DRAINING land promote WARMTH?

A. Because it diminishes evaporation; in consequence of which less heat is abstracted from the earth.

Q. Why does CULTIVATION increase the WARMTH of a country?

A. 1st--Because hedges and belts of trees are multiplied;

2ndly--Because the land is better drained;

3rdly--Because the land is dug and ploughed; and

4thly--Because the vast forests are cut down.

Q. Why do HEDGES and BELTS of TREES promote WARMTH?

A. Because they retard evaporation, by keeping off the wind.

Q. If belts of trees promote WARMTH, why do FORESTS produce COLD?

A. 1st--Because they detain and condense the passing clouds:

2ndly--They prevent the access of both wind and sun:

3rdly--The soil of forests is always covered with long damp grass, rotting leaves, and thick brushwood: and

4thly--There are always many hollows in every forest full of stagnant water.

Q. Why do LONG GRASS and ROTTING LEAVES promote COLD?

A. Because they are always damp; and the evaporation which they promote, is constantly absorbing heat from the earth beneath.

Q. Why do DIGGING and PLOUGHING help to make a country WARM?

A. Digging and ploughing help to pulverize the soil, by admitting air into it, and this increases its mean temperature.

Q. Why are FRANCE and GERMANY WARMER now, than when the vine would not ripen there?

A. Chiefly because their vast forests have been cut down; and the soil is better drained and cultivated.

Q. What becomes of the WATER of PONDS and TUBS in summer-time?

A. Ponds and tubs in summer-time are often left dry, because their water is evaporated by the air.

Q. How is this EVAPORATION PRODUCED and carried on?

A. The air contains heat, and changes the surface of the water into vapour; this vapour (blending with the air) is soon wafted away; while fresh portions of air blow over the water, and produce a similar evaporation; till the pond or tub is left quite dry.

Q. Why are the WHEELS of some machines kept CONSTANTLY WET with WATER?

A. To carry off the heat (arising from the rapid motion of the wheels) by evaporation, as soon as it is developed.

Q. Why is MOULD HARDENED by the SUN?

A. Because (when the moisture of the mould has been evaporated by the sun) the earthy particles come into closer contact, and the mass becomes more solid.

Q. Show the WISDOM of GOD in this arrangement.

A. If the soil did not become crusty and hard in dry weather, the heat and drought would penetrate the soil, and kill both seeds and roots.

Q. Why is TEA cooled FASTER in a SAUCER than in a cup?

A. Because evaporation is increased by increasing the surface; and as tea in a saucer presents a much larger surface to the air, its heat is more rapidly carried off by evaporation.

(The subject of "convection" will be treated of in a future chapter, and would scarcely be understood in this place.)

Q. Why is not the VAPOUR of the SEA SALT?

A. Because the salt is always left behind, by the process of evaporation.

Q. Why does a WHITE CRUST appear (in hot weather) upon CLOTHES wetted by sea water?

A. The white crust is the salt of the water left on the clothes by evaporation.

Q. Why does this WHITE CRUST always DISAPPEAR in WET weather?

A. In wet weather the moisture of the air dissolves the salt; and, therefore, it no longer remains visible.

Q. Why should NOT persons, who take violent exercise, WEAR very THICK CLOTHING?

A. When the heat of the body is increased by exercise, perspiration reduces the heat (by evaporation) to a healthy standard: as thick clothing prevents this evaporation, and confines the heat and perspiration to the body, it is injurious to health.