The  exciting  fluid  consists  of  potassium  bichromate  (9  parts),  sulphuric
acid  (25  parts),  and  water  (66  parts).  Polarization  is  prevented  through
the  reducing  action  of  the  liberated  hydrogen  on  the  chromic  oxide.*

•The  chemiciil  nctiun  in  Iht-  (.■xciliii;;   soliit
+  Ki^SO*  f  HsO  I  feHsSO*.     When  in  action  it

(a)  3Zn  +  3{HBS04)    -  aZnSO, -I  3Ha.
(b)  ac:ri)s  +  3H8  -  CrsOa  +  3H.O.
(c)  Cr^h  '  3(Hs.SO,).-  Cr,(SO.)«-t-3HrfJ.
ELECTRICITY.  loi

The  E.M.F.  of  this  coll  is  about  2  volts.  It  loses  in  intensity  when
ised  for  long  periods,  but  regains  it  when  allowed  to  remain  out  of
.ction  for  a  time.
The  Leclanch£  Cell  (Fig.  66). — ^This  consists  of  an  inner  porous  earthen
up  which  contains  a  carbon  plate  packed  in  fragments  of  coke  and
oanganese  dioxide.  The  top  is  covered  with  a  layer  of  pitch.  The
tositive  plate  is  a  rod  of  zinc,  which  dips  in  an  exciting  fluid  consisting
>f  a  saturated  solution  (6  ounces  to  a  quart  jar)  of  sal  ammoniac  con-
ained  in  the  outer  glass  jar.  Through  the  action  of  the  salt  on  the  zinc
.  double  chloride  of  zinc  and  ammonium  is  formed,  while  ammonia  and
lydrogen  gases  are  liberated  at  the  carbon  pole.  The  hydrogen  combines
nth   the    oxygen    evolved   by   chemical   action  fig.«6

rom  the  depolarizer  (manganese  dioxide),  and
snns  water.  If  electric  action  is  continued  for
oo  long  a  time,  excessive  quantities  of  hydrogen
oUect  and  polarization  ensues,  for  the  oxygen  is
;ot  developed  with  sufficient  rapidity ;  hence
hort  periods  of  rest  are  necessary.  The  great
opularity  of  this  cell  may  be  judged  from  the
anety  of  forms  in  which  it  appears  on  the  market,
t  does  not  require  renewing  for  months,  or  even
ears.  The  E.M.F.  of  this  cell  is  about  i.
