Am.  Jour.  Pharm.  \ 
Feb.  1, 1871.  J 
Lecture  Experiments. 
61 
million  parts  (2^5  milligramme  in  a  litre)  the  red  color  is  still  very 
plain,  and  silk  immersed  in  the  solution  is  colored  light  red  in  the 
space  of  a  quarter  of  an  hour.  If  the  dilution  is  still  further  continued, 
it  is  shown  that  in  case  the  proportion  be  1  part  dye  to  100  million 
parts  water  (yi^  mgr.  in  a  litre)  the  limit  is  attained  to  which  the 
coloration  is  perceptible.  Thin  layers  of  this  solution  appear  in  fact 
quite  colorless,  and  it  is  necessary  to  look  through  thicker  layers  (of 
about  metre)  or  to  observe  the  surface  of  the  solution  half  in  trans- 
mitted half  in  reflected  light,  in  order  clearly  to  recognize  the  tint. 
It  is  interesting  to  hang  a  thread  of  silk  in  a  volume,  not  too  small, 
of  this  almost  colorless  solution.  After  24  hours  the  thread  appears 
quite  plainly  and  more  deeply  colored  than  the  solution.  In  the 
light  of  this  phenomenon  we  cannot  doubt  that  in  the  bosom  of  the 
apparently  tranquil  solution  there  are  currents,  in  consequence  of 
which  the  colored  water-molecules  one  after  another  pass  over  the 
thread  at  rest.  The  observations  here  made  point  to  a  vibratory 
movement  of  the  molecules,  such  as  philosophers  are  compelled  to 
accept  as  a  conclusion  after  proceeding  upon  different  courses  of  rea- 
soning. 
If  instead  of  the  salt  of  rose  aniline  we  experiment  with  one  of  its 
numerous  derivatives,  we  observe  a  power  of  coloration  quite  similar. 
The  experiment  was  tried  particularly  with  aethyl-violet  and  with 
iodine  green.  In  both  cases  the  tint  was  perceptible  up  to  a  dilution 
of  100  million  parts,  and  both  solutions  fixed  upon  silk  after  a  long 
time  a  weak  but  perceptible  color.  Both  of  the  last  mentioned  dyes 
are  less  suitable  for  this  determination  of  limit  to  tinctorial  power 
than  the  salts  of  rose  aniline,  since  violet  and  green  in  the  dilute 
condition  lie  near  the  colors,  which  are  observed  in  looking  through 
notable  layers  of  pure  water. 
•1,    Formation  of  nitric  acid  hy  the  co7nbustion  of  hydrogen  in  the  air. 
It  is  well  known  that  in  the  explosion  of  a  mixture  of  hydrogen  and 
air  in  the  eudiometer,  traces  of  oxides  of  nitrogen  are  always  formed, 
and  that  merely  by  the  burning  o,f  hydrogen  in  the  air  a  little  nitro- 
gen is  always  oxidised.  Lately  I  have  observed  this  phenomenon 
under  circumstances  which  make  their  communication  perhaps  not 
without  value. 
In  case  of  a  popular  lecture  the  experiment  of  the  formation  of 
water  should  be  performed  on  quite  a  large  scale.  To  this  end  the 
two  gases  are  brought  together  in  a  glass  balloon  of  10  litres  capa- 
