210  Mr.  D.  Vaughan  on  the  Origin  of  Malaria. 
to  correspond  to  those  malarious  emanations  which  marshes  are 
ready  to  send  forth  in  obedience  to  the  elevation  of  temperature. 
From  the  limited  scale  on  which  they  are  produced  on  our 
globe,  these  volatile  oils  could  have  little  influence  on  the  con- 
dition of  human  health  if  they  were  uniformly  diffused  through 
the  atmosphere,  or  even  if  they  entered  it  by  vegetable  exhala- 
tion alone.  But,  from  their  slight  solubility  in  water,  they  are 
collected  during  rains  into  low  marshes  ;  and  under  the  influ- 
ence of  heat  the  air  in  these  localities  must  be  filled  with  the 
vapours  of  the  volatile  organic  matter  produced  on  the  high  lands 
around  them.  The  course  of  nature  in  this  case  is  very  similar 
to  that  exhibited  on  a  small  scale  in  obtaining  volatile  oils  or 
perfumes  from  different  parts  of  plants  by  aqueous  distillation. 
On  distilling  with  water  the  vegetable  products  in  which  they 
are  contained,  the  essential  oils  pass  over  with  the  steam  and 
are  obtained  by  condensation  in  a  pure  or  in  a  concentrated  form. 
Though  it  is  not  always  necessary  that  they  should  dissolve  in 
the  fluid  before  volatilizing,  yet  the  solution  generally  takes 
place;  and  it  contributes  to  the  success  of  the  operation  in 
cases  where  the  essential  oils  form  but  a  small  part  of  the  mate- 
rials from  which  they  are  ^manufactured.  In  obtaining  the  fer- 
ment oils,  the  leaves  and  other  parts  of  plants  capable  of  afford- 
ing them  require  to  be  steeped  in  water  for  some  days ;  and 
distillation  is  not  performed  until  the  odour  of  the  fluid  gives 
indication  that  the  oils  have  been  generated.  The  mode  gene- 
rally adopted  for  obtaining  these  volatile  principles  from  vege- 
table matter  depends  on  their  slight  solubility  in  water,  and  on 
the  manner  in  which  they  are  affected  by  an  increase  of  tempe- 
rature. 
Prom  these  results  it  is  easy  to  trace  the  inevitable  course  of 
similar  operations  transpiring  in  nature  on  a  far  more  extensive 
scale.  Of  the  rain  which  falls  during  wrarm  seasons,  a  large 
part  comes  into  repeated  contact  with  numerous  vegetable  forms 
as  it  descends  along  the  leaves  and  branches  of  trees  or  moves 
through  the  flowers  and  herbage  of  verdant  fields ;  so  that  on 
reaching  the  valleys  the  wrater  must  be  contaminated  with  the 
essential  oils  of  living  plants,  and  with  the  ferment  oils  of  their 
decaying  parts.  In  this  manner  a  marsh,  an  enclosed  lake,  or 
a  pond  becomes  the  receptacle  of  a  large  portion  of  the  volatile 
organic  matter  generated  within  the  basin  from  which  its  waters 
are  collected.  The  volatile  oils  which,  on  the  occurrence  of  cool 
and  heavy  rains,  are  thus  concentrated  from  a  wide  area  into  a 
marsh,  change  into  vapour  as  the  water  becomes  warm ;  and 
they  will  be  most  ready  to  contaminate  the  air  over  such  perma- 
nent abodes  of  moisture  during  the  intense  heat  of  summer. 
The  relation  which  I  have  traced  between  the  processes  of 
