﻿932 
  

  

  Mr. 
  A. 
  Ferguson 
  on 
  the 
  Forces 
  acting 
  on 
  a 
  

  

  As, 
  under 
  ordinary 
  circumstances, 
  the 
  values 
  of 
  m 
  range 
  

   from 
  2 
  to 
  6 
  grams, 
  a 
  super-sensitive 
  balance 
  is 
  not 
  a 
  deside- 
  

   ratum 
  • 
  it 
  is 
  extremely 
  difficult 
  to 
  keep 
  such 
  a 
  balance 
  at 
  rest 
  

   for 
  any 
  length 
  of 
  time, 
  whilst 
  the 
  precaution 
  just 
  described 
  

   is 
  absolutely 
  essential 
  to 
  the 
  consistency 
  of 
  the 
  results. 
  Re- 
  

   membering 
  then 
  that, 
  as 
  equation 
  (xviii.) 
  shows, 
  the 
  value 
  

   of 
  m 
  is 
  approximately 
  proportional 
  to 
  T, 
  it 
  will 
  be 
  seen 
  that 
  a 
  

   balance 
  which 
  just 
  turns 
  with 
  2 
  or 
  3 
  milligrams 
  will 
  give 
  the 
  

   value 
  of 
  T 
  to 
  at 
  least 
  1 
  part 
  in 
  1000, 
  as 
  far 
  as 
  the 
  weighings 
  

   are 
  concerned. 
  

  

  Three 
  liquids 
  were 
  selected 
  for 
  examination 
  — 
  benzene, 
  

   absolute 
  alcohol, 
  and 
  water. 
  The 
  experiments 
  of 
  Magie* 
  

   have 
  shown 
  that 
  these 
  three 
  liquids, 
  in 
  contact 
  with 
  glass, 
  

   have 
  zero 
  contact-angles. 
  In 
  the 
  case 
  of 
  the 
  last-mentioned 
  

   liquid, 
  tap-water 
  was 
  used, 
  as 
  the 
  laboratory 
  supply 
  is 
  

   remarkably 
  free 
  from 
  dissolved 
  substances, 
  and 
  experience 
  

   has 
  shown 
  that 
  tap-water 
  is, 
  on 
  the 
  whole, 
  more 
  likely 
  to 
  be 
  

   free 
  from 
  surface 
  impurities 
  than 
  a 
  sample 
  of 
  distilled 
  water 
  

   which 
  has 
  been 
  stored 
  in 
  a 
  bottle. 
  

  

  We 
  now 
  proceed 
  to 
  discuss 
  the 
  numerical 
  values 
  obtained. 
  

   The 
  figures 
  given 
  below 
  are 
  in 
  all 
  cases 
  in 
  c.G.s. 
  units, 
  whilst 
  

   temperatures 
  are 
  given 
  in 
  degrees 
  centigrade. 
  For 
  the 
  

   sake 
  of 
  comparison 
  the 
  surface 
  tensions 
  are 
  in 
  all 
  cases 
  

   reduced 
  to 
  15°, 
  from 
  the 
  known 
  values 
  of 
  the 
  temperature 
  

   coefficients* 
  

  

  Benzene 
  (chemically 
  pure). 
  

   Eadius 
  of 
  sphere 
  = 
  7*321 
  cm. 
  

  

  e. 
  

  

  m. 
  

  

  a\ 
  

  

  T 
  - 
  

  

  T 
  ls 
  . 
  

  

  d. 
  

  

  o 
  

  

  16 
  

  

  2-565 
  

  

  •0S383 
  

  

  29-14 
  

  

  29-29 
  

  

  + 
  •05 
  

  

  14-2 
  

  

  2-585 
  

  

  •03410 
  

  

  29-37 
  

  

  29-25 
  

  

  +-oi 
  

  

  15-3 
  

  

  2-573 
  

  

  •03393 
  

  

  29-22 
  

  

  29-26 
  

  

  + 
  •02 
  

  

  14-2 
  

  

  2-582 
  

  

  •03405 
  

  

  29-33 
  

  

  29-21 
  

  

  -•03 
  

  

  143 
  

  

  2-580 
  

  

  •03402 
  

  

  29-31 
  

  

  29-21 
  

  

  -03 
  

  

  ]4-2 
  

  

  2-585 
  

  

  •03410 
  

  

  29-37 
  

  

  29-25 
  

  

  + 
  01 
  

  

  ]4o 
  

  

  2-582 
  

  

  •03405 
  

  

  29-33 
  

  

  29-26 
  

  

  + 
  •02 
  

  

  14-2 
  

  

  2-583 
  

  

  •03406 
  

  

  29-34 
  

  

  29-22 
  

  

  -•02 
  

  

  Mean 
  value, 
  T 
  15 
  = 
  29'24 
  

  

  dynes 
  

  

  cm. 
  

  

  Greatest 
  divergence 
  from 
  mean 
  value 
  = 
  *17 
  per 
  cent. 
  

   * 
  Phil. 
  Mag. 
  vol. 
  xxvi. 
  p. 
  162 
  (1888). 
  

  

  