ON  DROPS. 
523 
difficulty  of  obtaining  a  standard  drop — a  difficulty  which  is 
still  more  increased  by  the  knowledge  that  even  when  the  same 
vessel  and  liquid  are  used  the  differences  are  almost  as  great  as 
those  already  cited.  Thus,  in  experimenting  on  water  with 
ounce  vials,  Parrish  found  that  in  seven  trials  the  number  of 
drops  required  to  make  up  a  fluidrachm  varied  between  thirty- 
two  and  sixty-five.  The  necessity,  therefore,  for  Professor 
Guthrie's  investigations  is  at  once  apparent, 
Ttte  Professor  sets  out  by  defining  a  drop  as  any  mass  of 
liquid  matter  whose  form  is  visibly  influenced  towards  the 
spherical  by  the  attraction  of  its  parts,  and  whose  sensible  mo- 
tion or  tendency  is  towards  the  earth.  He  then  goes  on  to  ex- 
clude from  consideration  drops  which  are  formed  under  indefi- 
nite, or  at  any  rate  unmeasurable,  circumstances,  such  as  rain 
drops,  including  only  those  that  are  formed  under  fixed  and 
determinable  conditions.  The  drops  which  form  the  subject  of 
experiment  may,  therefore,  be  defined  as  masses  of  liquid  col- 
lected or  held  together  by  the  attraction  of  their  parts,  and 
separated  from  each  other  by  the  attraction  of  gravitation. 
This  definition  includes  upward-moving  drops,  which  are  formed 
when  a  heavy  liquid,  such  as  water,  is  carefully  poured  into  a 
bottle  containing  a  lighter  one,  like  petroleum,  the  bottom  of 
the  vessel  always  holding  a  certain  quantity,  which  gradually 
"  drops  up"  to  the  surface. 
The  size  of  a  drop  generally  depends  on,  and  is  influenced 
by  at  least  four  conditions  : — 1.  The  self-attraction  of  the  drop- 
generating  liquid.  2.  Its  adhesion  to  the  matter  on  which  the 
drop  is  formed.  3.  The  shape  of  this  matter.  4.  The  physi- 
cal relations  existing  between  the  matter  on  which  the  drop  is 
formed,  the  liquid  constituting  the  drop  itself,  and  the  medium 
through  which  it  passes. 
Denoting  the  three  states  of  matter  by  the  letters  S  (solid), 
L  (liquid),  and  G  (gaseous),  and  considering  the  symbols  in  the 
order  in  which  they  are  written  to  denote  respectively  the  mat- 
ter from  which  the  dropping  takes  place,  the  drop  and  the  me- 
dium, we  get  a  convenient  notation.  There  are  eight  varia- 
tions of  these  conditions,  but  only  three  of  them  are  possible. 
S  L  L,  when  from  a  solid  a  liquid  drops  through  a  liquid. 
S  L  G,  when  from  a  solid  a  liquid  drops  through  a  gas. 
