Mr.  D.  Vaughan  on  the  Origin  of  Malaria.  211 
nature  and  of  art  for  concentrating  into  a  limited  space  the 
diffusible  oils  of  plants,  does  not  require  that  the  water  of  a 
marsh  should  be  subjected  to  a  boiling  heat.     This  will  appear 
evident  from  well-known  facts  and  principles  respecting  vapori- 
zation; and   without   insisting   on    the   rigorous   accuracy   of 
Dalton's  law,  we  may  use  it  to  determine  approximately  the 
amount  of  these  volatile  oils  which  at  a  certain  temperature  can 
exist  as  vapour  in  a  given  volume  of  air,  or  rather  in  a  given 
space.     This  may  be  estimated  theoretically  from  the  boiling- 
point  of  the  oil  and  from  the  density  of  its  vapour.     It  may  be 
thus  found  that  eight  thousand  parts  of  air  at  the  temperature  of 
50    F.  would  be  capable  of  holding  as  vapour  one  part  of  the 
oil  of  wintergreen,  two  parts  of  the  oil  of  meadow-weed,  or  six- 
teen parts  of  the  oil  of  turpentine ;  but  the  same  air  could  hold 
double  the  amount  of  these  vapours  at  75°F.,  and  four  times  as 
much  at  100°  F.     With  regard  to  the  ferment  oils,  the  slow  rate 
at  which  they  pass  over  with  the  steam  during  their  distillation 
is  a  sufficient  indication  that  it  requires  but  a  very  small  portion 
of  their  vapours  to  saturate  the  air,  even  during  our  warmest 
summers. 
From  further  inquiries  as  to  the  origin  of  these  organic 
bodies,  their  presence  in  low  marshes  will  appear  still  more 
dependent  on  heat ;  and  more  evidence  is  thus  obtained  of  their 
identity  with  malaria.  It  is  in  the  warmest  climates  affording  the 
proper  conditions  for  vegetation  that  essential  oils  are  produced 
in  the  greatest  abundance  and  ferment  oils  are  most  rapidly 
developed  by  decaying  vegetable  matter.  Accordingly  in  these 
regions  the  air  over  marshes  will  most  frequently  receive  almost 
the  full  amount  of  organic  vapours  which  its  high  temperature 
enables  it  to  sustain.  But  the  restless  condition  of  the  air 
often  prevents  it  from  remaining  over  these  localities  long 
enough  to  be  largely  impregnated  or  poisoned  by  their  exha- 
lations; and  the  trade-winds  contribute  much  to  avert  the 
insalubrity  which  heat  and  moisture  are  ever  ready  to  produce 
in  tropical  climates. 
In  marshes  surrounded  by  high  grounds  the  air  is  consider- 
ably impeded  in  its  movements,  and  is  thus  caused  to  imbibe  a 
larger  quantity  of  their  volatile  vapours.  To  the  calmness  from 
which  such  evils  arise,  the  presence  of  water  in  a  valley  contri- 
butes in  an  indirect  manner.  From  the  absorption  of  heat  in 
the  formation  of  aqueous  vapour,  the  lowest  air  in  such  localities 
would  have  a  specific  gravity  higher  than  its  position  would  call 
for,  and  it  would  be  accordingly  less  sensitive  to  those  forces 
which  occasion  gentle  winds.  ^Numerous  trees  also  tend  much 
to  the  quiescent  condition  of  the  air  in  valleys  and  marshes ; 
and  it  is  chiefly  on  this  account  that  the  destruction  of  forests 
P2 
