212 
Mr.  D.  Vaughan  on  the  Origin  of  Malaria. 
has  contributed  so  much  to  check  the  career  of  intermittent 
fever  in  the  New  World.  It  is  partly  in  consequence  of  the 
storms  with  which  they  are  accompanied  that  excessive  rains 
arrest  for  a  time  the  effects  of  malaria;  but  the  mitigation  of 
the  evil  also  depends  on  the  excessive  dilution  of  the  volatile 
oils  with  water,  and  their  diminished  liability  to  escape  into 
the  air. 
In  dry  valleys,  in  marshes  which  have  been  drained,  and  in 
lakes  having  an  outlet,  the  excessive  accumulation  of  the  vapour 
of  malaria  is  in  a  great  measure  prevented ;  but  the  evils  which 
it  is  capable  of  inflicting  are  generally  transferred  to  other  places. 
Though  volatile  oils  oxidize  and  become  inert  in  the  air,  they 
are  slow  to  change  their  characters  in  water;  and,  accordingly, 
a  river  which  drains  lands  teeming  with  verdure  and  bearing  a 
luxuriant  vegetation  must  be  impregnated  with  the  volatile 
organic  matter  which  they  supply,  and  which  is  ready  to  escape 
into  the  air  when  favoured  by  the  influence  of  heat  and  evapora- 
tion. It  is  on  this  account  that  the  rivers  of  Italy  contribute 
much  to  the  insalubrity  of  the  lands  through  which  they  pass ; 
and  many  marshes  along  their  banks  exhibit  the  effects  of 
malaria  imported  from  distant  localities,  and  surrendered  to  the 
air  by  the  water  which  is  imprisoned  and  evaporated  in  stagnant 
pools.  Rivers  also  find  the  conditions  for  diffusing  their  volatile 
organic  matter  when  they  pass  through  lakes,  or  when  they 
spread  their  waters  over  the  deltas  which  are  so  generally  formed 
at  their  mouths  in  tideless  seas. 
As  volatile  oils  slowly  escape  from  the  water  or  change  their 
characters  by  oxidation,  there  must  be  a  limit  to  the  distance 
to  which  their  poisonous  characters  can  be  transmitted  by  rivers. 
An  instance  of  the  effects  which  these  causes  produce  may  be 
found  in  the  case  of  the  Nile,  which  in  the  latter  part  of  its 
course  runs  over  a  thousand  miles  without  receiving  a  single 
tributary.  Whatever  volatile  poisons  may  have  contaminated 
its  waters  in  Equatorial  Africa,  must  have  been  either  expelled 
by  heat  or  rendered  inert  by  oxidation  during  its  long  journey 
to  the  Mediterranean.  Accordingly  its  floods  do  not  prevent 
Egypt  from  enjoying  a  comparatively  healthy  climate.  This 
boon  may  be  partly  ascribed  to  the  dryness  of  the  air  and  the 
unfrequency  of  rains  in  Egypt;  but  Bussorah,  in  a  region 
equally  dry  and  free  from  rains,  is  rendered  extremely  pesti- 
lential by  the  inundations  of  the  Euphrates;  and  the  streams 
which  descend  along  the  verdant  vales  on  the  southern  side  of 
the  Elburz  mountains,  give  to  the  city  of  Teheran  a  degree  of 
insalubrity  which  cannot  be  ascribed  to  the  luxuriance  or  to  the 
decay  of  vegetation  in  its  immediate  vicinity. 
It  is  probable  that  the  various  volatile  oils  under  consideration 
