242       Dr.  C.  R.  A.  Wright  on  the  Relations  between  the 
— in  other  words,  that  the  conceptions  involved  in  this  hypo- 
thesis are  both  unnecessary  and  insufficient. 
In  pursuance  of  this  object  it  is  clearly  necessary,  first,  to 
enumerate  the  fundamental  facts  in  language  that  clearly  ex- 
presses them  without  at  the  same  time  involving  any  theoretical 
views  whatever,  and  then  to  enunciate  the  theory  and  show  how 
far  it  accounts  for  these  facts.  In  most  writings  on  the  subject 
it  unfortunately  happens  that  terms  such  as  atom,  molecule, 
structure,  &c.  are  employed  in  stating  fundamental  data,  these 
terms  necessarily  involving  the  ideas  and  notions  of  the  very 
hypothesis  under  consideration;  to  avoid  the  petitio  principii 
thus  rendered  imminent,  it  is  desirable  to  eliminate  such  terms 
altogether  from  the  language  employed  in  stating  the  funda- 
mental facts,  and  the  way  in  which  they  are  expressed  and  alluded 
to  by  the  ordinary  symbols. 
1.  The  volume  occupied  by  1  gramme  of  hydrogen  under 
given  circumstances  as  to  temperature  and  pressure  in  any  expe- 
riment is  termed  a  "Relative  Vapour-volume  Unit,  or  more 
briefly,  a  "  Volume. ,J 
2.  The  specific  gravities  of  vapours  (vapour-densities)  with 
respect  to  hydrogen  mean  the  numbers  of  grammes  that  a  volume 
of  the  gases  or  vapours  weighs,  the  pressure  and  temperature  being 
respectively  such  that  the  compounds  in  question  are  aeriform. 
Thus  the  volume  occupied  by  1  gramme  of  hydrogen  atO°  and 
760  millims.  is  11  "2  litres;  11  "2  litres  of  oxygen  at  this  pres- 
sure and  temperature  weigh  16  grammes ;  hence  16  is  the  spe- 
cific gravity  of  oxygen. 
The  volume  occupied  by  1  gramme  of  hydrogen  at  temperature 
t°  and   pressure  p  millims.  is  11*2  x  (1  +  0*00365  x  t)  x  . 
This  volume  of  water  (the  pressure  and  temperature  being  such 
that  the  water  is  aeriform)  weighs  9  grammes ;  hence  the  vapour- 
density  of  water  is  9. 
3.  A  homogeneous  body,  when  submitted  to  analytical  pro- 
cesses, is  either  capable  of  furnishing  simultaneously  more,  or 
not  more,  than  one  substance  dissimilar  in  properties  to  the 
original  body.  In  the  first  case  the  body  is  said  to  be  a  com- 
pound, to  be  composed  of  the  several  substances  which  it  can  thus 
furnish  on  analysis;  in  the  latter  case  it  is  called  an  element; 
and  if  it  furnish  one  substance  dissimilar  to  the  original  body, 
the  original  substance  is  said  to  be  an  allotropic  modification  of 
this  element. 
Thus  ozone  analyzed  by  heat  or  by  induction-sparks  yields 
but  one  body  dissimilar  from  ozone ;  this  new  product  is  termed 
oxygen,  and  ozone  is  said  to  be  an  allotropic  modification  of  the 
element  oxygen  :  oxygen  itself,  never  having  been  analyzed  into 
any  two  substances,  is  therefore  termed  an  element. 
