A. If the iron be _very hot indeed_, the _bottom_ of the drop is turned into _vapour, before the drop can evaporate_; and the vapour thus formed _buoys the drop up_, without allowing it to touch the iron at all. Q. _Why does it ROLL?_ A. The _current of air_ (which is always passing over the heated surface) _drives it along_. Q. _Why does a LAUNDRESS put a little SALIVA on an IRONING-BOX to know if it be hot enough?_ A. If the saliva _sticks to the box and is evaporated_, the box is _not_ hot enough; but if the saliva _runs along the box_, it _is_. Q. _Why is the BOX HOTTER if the saliva RUNS ALONG THE BOX, than if it adheres to it till it is evaporated?_ A. If the saliva _runs along the box_, the iron is hot enough to _convert the bottom of the drop_ of spittle into _vapour_; but if the saliva _will not roll_, the box is _not_ hot enough to convert the bottom of the drop of spittle into vapour. 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?_