6 
ME.  GASSIOT  ON  THE  STEATIEICATIONS  AND  DAEK  BAND  IN 
a mass  of  mercury  as  is  contained  in  a tube  of  upwards  of  an  inch  internal  diameter  and 
34  or  35  inches  in  length,  can  fully  appreciate. 
Mr.  Welsh  kindly  superintended  at  Mr.  Casella’s  the  filluig  of  a tube  on  his  prin- 
ciple (Plate  I.  fig.  3) : the  process  used  by  this  gentleman,  which  avoids  all  the  difficulty 
of  boiling,  has  been  fully  described  in  the  Philosophical  Transactions  for  1856*.  The 
only  difierence  in  the  present  instance  is  the  previously  inserting  the  platinum  wires  at  a 
and  the  tubes  being  hermetically  sealed  at  (Z,  about  4 inches  from  the  wire  h. 
The  result  realized  to  the  fullest  extent  our  expectations : in  this  tube  the  bands  were 
clear,  well-defined  and  distinct ; and,  provided  care  be  taken  in  manipulating  v^ith  the 
coil,  so  as  always  to  send  the  discharge  in  the  same  direction,  the  platinum  coating  is 
deposited  on  the  portion  of  the  tube  surrounding  the  negative  wire,  but  none  at  or  near 
the  positive. 
21.  With  this  tube  I obtained  the  stratification  by  the  discharge  of  the  electrical 
machine.  This  experiment  was  first  made  by  me  on  the  28th  of  December  last,  in  the 
Theatre  of  the  London  Institution,  in  the  presence  of  Professor  Tyxdall  and  Mr.  Maloxe, 
connexion  being  made  from  one  platinum  wire  to  the  conductor  of  a 4-feet  electrical 
machine,  while  the  other  wire  was  attached  to  the  rubber.  On  turning  the  plate  the 
discharge  took  place  through  the  tube,  filling  it  with  the  usual  phosphorescent  light, 
but  exhibiting  the  strise  very  distinctly. 
22.  The  cause  of  the  dissimilarity  in  the  discharge  of  A,  B and  C,  and  of  D and  E, 
although  respectively  of  similar  dimensions,  did  not  arise  from  any  dissimilarity  in  the 
mode  of  obtaining  the  vacuum,  but  from  the  glass  tubes  not  ha\ing  been,  previous  to 
the  insertion  of  the  mercury,  equally  well  cleaned  or  equally  well  deprived  of  moistime, 
a circumstance  which  Mr.  Welsh  has  shown  is  indispensable  in  the  construction  of  a 
good  barometer,  and  this  cleansing  can  only  be  obtained  by  mechanical  means ; in  C the 
moisture  is,  as  I have  observed,  perceptible,  and  in  this  the  discharge  is  most  imperfect. 
With  a tube  prepared  on  Mr.  Welsh’s  principle,  and  a moderate-sized  Kehmeoeff’s 
coil  excited  by  a single  cell  of  Gkove’s  nitric  acid  battery,  the  phenomena  of  the  strati- 
fied discharge  can  be  seen  and  examined  with  ease,  without  the  trouble  and  uncertain 
manipulation  of  the  air-pump,  or  using  phosphoric  and  other  vapoiu’s. 
23.  My  next  experiments  were  made  with  apparatus  the  form  and  size  of  which  were 
different  to  those  I had  already  used,  but  which  had  been  made  previous  to  experi- 
menting mth  the  last-named  tube,  fig.  3.  Fig.  4,  Plate  I.,  represents  an  apparatus 
with  moderate-sized  thermometer  tubing,  having  wires,  a and  inserted  as  before,  with 
stopcock  C for  exhausting  by  air-pump : in  this  apparatus  the  discharge  is  diA'ided  into 
alternate  Avhite  and  dark  bands,  having  the  appearance  of  ruled  lines. 
24.  Fig.  5 represents  a similar  apparatus,  but  of  a pear  shape,  3 inches  mternal 
diameter ; the  wires  are  5 inches  apart  from  point  to  point.  Although  great  care  had 
been  taken  in  boihng  this  large  vessel  of  mercury,  containing  about  10  lbs.  of  the  metal, 
the  vacuum  was  not  found  to  be  perfect,  and  moreoA  cr  the  mercury  remained  deposited 
* vol.  cxlvi.  p.  507. 
