第 33 节
作者:精灵王      更新:2021-04-30 17:23      字数:9317
  rapid succession to the         surface all over the vessel; and this process goes
  on    uninterruptedly as long as the rays of the sun are uninterrupted。
  〃Now   these   globules   consist   of   PURE   OXYGEN;   given   out   by   the
  plants     under the stimulus of light; and to this oxygen the animals in the
  tank owe their life。        The difference between the profusion of                oxygen…
  bubbles   produced   on   a   sunny   day;   and   the   paucity   of   those   seen   on   a
  dark   cloudy   day;   or   in   a   northern   aspect;   is   very marked。〃       Choose;
  therefore; a south or east window; but draw down                   the blind; or throw a
  handkerchief       over   all  if  the  heat   become     fierce。     The    water    should
  always   feel   cold   to   your   hand;   let   the temperature   outside   be   what   it
  may。
  Next;   you   must   make   up   for   evaporation   by   FRESH   water   (a   very
  little will suffice); as often as in summer you find the water in                your vase
  sink below its original level; and prevent the water from                 getting too salt。
  For the salts; remember; do not evaporate with                the water; and if you left
  the vase in the sun for a few weeks; it           would become a mere brine…pan。
  But how will you move your treasures up to town?
  The   simplest   plan   which   I   have   found   successful   is   an   earthen   jar。
  You   may  buy  them  with   a   cover   which   screws on   with   two   iron   clasps。
  If you do not find such; a piece of oilskin tied over the mouth is                 enough。
  But do not fill the jar full of water; leave about a             quarter of the contents
  in empty air; which the water may absorb;              and so keep itself fresh。        And
  any pieces of stone; or oysters;         which you send up; hang by a string from
  the   mouth;   that   they   may    not   hurt   tender   animals   by   rolling   about   the
  bottom。      With these      simple precautions; anything which you are likely
  to find will well      endure forty…eight hours of travel。
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  What if the water fails; after all?
  Then Mr。 Gosse's artificial sea…water will form a   perfect                substitute。
  You may buy the requisite salts (for there are more                  salts than 〃salt〃   in
  sea…water)      from    any   chemist    to  whom      Mr。   Gosse     has   entrusted    his
  discovery; and; according to his directions; make               sea…water for yourself
  One more hint before we part。            If; after all; you are not going        down
  to   the   sea…side   this   year;  and   have    no   opportunities     of   testing    〃the
  wonders of the shore;〃 you may still study Natural                 History in your own
  drawing…room; by looking a little into 〃the             wonders of the pond。〃
  I   am   not   jesting;   a   fresh…water   aquarium;   though   by   no   means      as
  beautiful as a salt…water one; is even more easily established。                   A    glass
  jar;   floored   with   two   or   three   inches   of   pond…mud   (which      should   be
  covered with fine gravel to prevent the mud washing up);                    a specimen of
  each of two water…plants which you may buy now at any                       good shop in
  Covent Garden; Vallisneria spiralis (which is said to                give to the Canvas…
  backed duck of America its peculiar richness of                flavour); and Anacharis
  alsinastrum;   that   magical   weed   which;        lately   introduced   from   Canada
  among timber; has multiplied; self… sown; to so prodigious an extent; that
  it   bid   fair;   a   few   years since;   to   choke   the   navigation   not   only   of   our
  canals   and   fen…   rivers;   but   of   the   Thames   itself: (34)   or;   in   default   of
  these;     some      of  the   more    delicate   pond…weeds;       such    as  Callitriche;
  Potamogeton   pusillum;   and;   best   of   all;   perhaps;   the   beautiful       Water…
  Milfoil     (Myriophyllium);       whose     comb…like     leaves    are  the   haunts     of
  numberless rare and curious animalcules:… these (in                themselves; from the
  transparency       of  their  circulation;    interesting    microscopic       objects)   for
  oxygen…breeding vegetables; and for              animals; the pickings of any pond;
  a minnow or two; an eft; a few             of the delicate pond…snails (unless they
  devour       your     plants    too      rapidly):        water…beetles;       of    activity
  inconceivable;   and   that      wondrous   bug   the   Notonecta;   who   lies   on   his
  back all day; rowing         about his boat…shaped body; with one long pair of
  oars; in search       of animalcules; and the moment the lights are out; turns
  head over       heels; rights himself; and opening a pair of handsome wings;
  starts    to fly about the dark room in company with his friend the water…
  beetle; and (I suspect) catch flies; and then slips back demurely                  into the
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  water with the first streak of dawn。           But perhaps the most          interesting of
  all the tribes of the Naiads; … (in default; of          course; of those semi…human
  nymphs   with   which   our   Teutonic        forefathers;   like   the   Greeks;   peopled
  each 〃sacred   fountain;〃)   …      are   the   little   〃water…crickets;〃   which   may  be
  found running   under         the pebbles;  or burrowing in little galleries in   the
  banks:     and     those 〃caddises;〃   which crawl   on the bottom in   the  stiller
  waters;     enclosed; all save the head and legs; in a tube of sand or pebbles;
  shells    or   sticks;   green    or   dead    weeds;    often    arranged     with    quaint
  symmetry; or of very graceful shape。              Their aspect in this state        may be
  somewhat uninviting; but they compensate for their youthful                    ugliness by
  the strangeness of their transformations; and often by                the delicate beauty
  of   the   perfect   insects;   as   the   〃caddises;〃  rising   to   the   surface;   become
  flying Phryganeae (caperers and sand… flies); generally of various shades
  of fawn…colour; and the water… crickets (though an unscientific eye may be
  able to discern but        little difference in them in the 〃larva;〃 or imperfect
  state)    change into flies of the most various shapes; … one; perhaps; into
  the   great   sluggish   olive   〃Stone…fly〃   (Perla   bicaudata);   another      into   the
  delicate   lemon…coloured   〃Yellow   Sally〃   (Chrysoperla              viridis);   another
  into   the   dark   chocolate   〃Alder〃   (Sialis   lutaria):     and   the   majority   into
  duns   and   drakes   (Ephemerae);   whose   grace   of          form;   and   delicacy   of
  colour;   give   them   a   right   to   rank   among   the   most   exquisite   of   God's
  creations; from the tiny 〃Spinners〃 (Ba 塼 is               or Chloron) of incandescent
  glass;   with   gorgeous   rainbow…coloured          eyes;   to   the   great   Green   Drake
  (Ephemera vulgata); known to all              fishermen as the prince of trout…flies。
  These animals; their        habits; their miraculous transformations; might give
  many      an  hour's    quiet    amusement       to   an  invalid;   laid   on   a  sofa;   or
  imprisoned in a        sick…room; and debarred from reading; unless by some
  such means;        any page of that great green book outside; whose pen is the
  finger     of God; whose covers are the fire kingdoms and the star kingdoms;
  and its leaves the heather…bells; and the polypes of the sea; and                 the gnats
  above the summer stream。
  I   said  just   now;    that  happy    was    the   sportsman     who    was    also   a
  naturalist。     And;     having    once   mentioned      these   curious    water…flies;    I
  cannot help going a little farther; and saying; that lucky is the                fisherman
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  who is also a naturalist。         A fair scientific knowledge of             the flies which
  he   imitates;   and   of   their   habits;   would   often     ensure   him   sport;   while
  other men are going home with empty creels。                     One would have fancied
  this a self…evident fact; yet I have never             found any sound knowledge of
  the    natural    water…flies    which     haunt    a   given      stream;    except