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 _r.e.t.a.r.ds 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_.