236  Law  Applicable  to  Gases  and  Liquids.  {Am-^-1^arm- 
A  GENERAL  LAW  APPLICABLE  TO  GASES  AND 
LIQUIDS. 
By  J.  Alfred  Wanklyn. 
One  of  the  results  flowing  from  the  work  upon  which  I  have  been 
engaged  for  many  years,  sometimes  alone  and  rometimes  in  com- 
pany with  my  colleagues,  Cooper  and  Johnstone,  is  the  unfolding  of 
a  great  generalization,  applicable  alike  to  gases  and  liquids,  which 
may  be  formulated  in  the  following  terms  : 
Heterogeneity  is  without  influence  upon  volume,  or  the  volume 
of  a  mixture  is  equal  to  the  sum  of  the  volumes  of  its  constituents 
separately  measured. 
Gases. — So  far  as  I  am  aware — although  the  reasoning  on  the 
simplest  examples  of  gas  analysis  involves  the  admission  of  the 
truth  of  this  generalization  as  applied  to  gases — the  generalization 
has  never  been  explicitly  set  out.*  It  is  not  by  any  means  manifest 
■a  priori,  but  is  founded  upon  very  wide  observation. 
Thirty-one  years  ago  it  fell  to  my  lot  to  place  on  record  in  the 
Trans,  of  the  Royal  Society  of  Edinburgh,  a  series  of  observations 
illustrative  of  the  great  truth  that  gaseous  mixtures  occupy  exactly 
the  same  volume  as  the  constituents  of  the  mixture. 
The  paper  has  been  frequently  quoted,  because  in  that  paper  Sir 
Lyon  Playfair  and  myself  announced  the  discovery  of  the  duplicate 
nature  of  nitric  peroxide,  which  is  N204  at  low  temperatures  and 
N02  at  high  temperatures.  The  completeness  and  the  striking 
character  of  the  evidence  bearing  upon  the  fundamental  nature  of 
gaseous  mixtures  has,  however,  escaped  attention  altogether,  and 
it  will  not  be  out  of  place  to  refer  to  it  in  a  detailed  manner.  The 
paper  bore  the  title  "  On  a  Mode  of  Taking  the  Density  of  Vapor 
of  Volatile  Liquids  at  Temperatures  below  the  Boiling-point,"  by 
Dr.  Lyon  Playfair,  C.B.,  F.R.S.,  and  J.  A.  Wanklyn,  F.R.S.E.,  and 
it  was  read  January  7,  1 861 . 
The  method  consisted  in  measuring  the  volume  occupied  by  a 
mixture  of  a  measured  quantity  of  a  permanent  gas,  and  an  ascer- 
tained weight  of  the  vapor  under  investigation  ;  and  our  paper 
contained  a  detailed  proof  that  such  mixtures  have  the  same 
volume  as  the  sum  of  their  constituent  volumes,  measured  separ- 
ately. We  proved  this  for  mixtures  of  alcohol  and  hydrogen,  for 
mixtures  of  ether  and  hydrogen,  for  mixtures  of  nitrate  of  ethyl 
and  nitrogen,  for  mixtures  of  nitric  acid  and  air,  and  lastly,  for  the 
