CHAPTER XII.

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.