Intelligence  and  Miscellaneous  Articles,  399 
In  any  case,  if  a  equals  the  number  of  vibrations  per  second  of  the 
fork,  and  b  equals  the  observed  number  of  undulations  in  a  single 
turn  cf  the  disk,  then  V  =— . 
b 
A  single  wave,  or  even  half  of  one,  is  sufficient  for  determination 
if  the  length  be  measured  in  degrees,  in  which  case  if  c?=  the  length 
of  one  wave,  in  degrees,  the  formula  will  stand  V=  ~^r.     If  the  ro- 
tation  be  very  rapid,  the  quickest  possible  touch  is  needed,  or  the 
undulations  will  return  and  confuse  each  other  ;  but  this  is  not 
troublesome  at  any  speed  1  have  been  able  to  obtain,  which  is  in  the 
neighbourhood  of  90  per  second. 
1  have  found  that  a  common  pocket  tuning-fork,  either  an  A  or  a 
C,  answers  very  nicely  with  one  of  india-rubber  fastened  as  before. 
So  far  as  I  now  know,  the  swiftest  motion  that  has  been  given  to  a 
disk  was  that  in  Foucault's  apparatus  for  showing  the  motion  of  the 
earth,  which  he  estimated  at  from  150  to  200  revolutions  per  second. 
Such  a  velocity  would  be  recorded  by  from  two  to  three  undulations 
of  a  C  fork,  making  512  vibrations  per  second.  If  it  would  not  be 
best  to  smoke  the  disk,  a  piece  of  white  paper  can  be  pasted  upon  it 
and  smoked  without  burning  ;  and  it  answers  every  purpose.  To  the 
certainty  and  ease  of  this  method  may  be  added  another  advantage, 
that  the  slightest  touch  needed  for  this  cannot  sensibly  retard  the 
motion  of  the  disk,  as  any  mechanical  fixture  attached  for  such  a 
purpose  must  do. — Silliman's  American  Journal  for  April  1872. 
PRELIMINARY  NOTE  ON  A  REMARKABLE  FACT  OBSERVED  ON  THE 
CONTACT  OF  CERTAIN  LIQUIDS  OF  VERY  DIFFERENT  SUPERFI- 
CIAL TENSIONS.       BY  G.  VAN  DER  MENSBRUGGHE. 
Whenever  a  liquid  of  strong  superficial  tension,  containing  gases  in 
solution,  is  brought  into  contact  with  a  liquid  of  feeble  tension,  there  is 
a  more  or  less  pronounced  disengagement  of  the  gases  dissolved  in  the 
former  liquid. 
This  principle,  which  I  publish  now  to  secure  date,  intending, 
however,  to  verify  it  in  detail  in  a  special  memoir,  can  be  demon- 
strated by  a  great  number  of  experiments.  Provisionally  I  will  only 
mention  a  few. 
I.  When  a  drop  of  alcohol  or  ether  is  introduced  into  distilled 
water  half  filling  a  phial  of  3  or  4  centims.  diameter  and  the  liquid 
agitated,  a  brisk  effervescence  ensues.  This  experiment  was  long 
since  described  by  M.  Duprez*,  but  without  explanation.  It  is  im- 
possible to  attribute  the  effervescence  to  air  introduced  by  the  agita- 
tion, since  neither  alcohol  or  ether  alone  nor  water  alone  gives  in 
this  respect  any  marked  result. 
The  experiment  succeeds  equally  well  with  benzole,  sulphide  of 
carbon,  creosote,  oil  of  turpentine,  olive,  lavender,  linseed,  or  colza 
oil,  petroleum,  oil  of  sweet  almonds,  &c.  One  has  even  merely  to 
agitate  distilled  water,  after  having  immersed  in  it  a  glass  rod  bear- 
*  Bull,  de  I' Acad.  Roy  ale  de  Belgique,  1838,  ser.  1.  vol.  v.  p.  402. 
