^TmsTs**  }  Preservation  of  Food.  497 
to  four  hours,  according  to  the  nature  of  the  article — if  it  is  compa- 
ratively dry,  as  peas,  beans  or  corn,  it  takes  much  longer  heating  than 
if  juicy,  like  tomatoes,  the  latter  being  one  of  the  easiest  articles  to 
preserve.  In  some  cases  a  small  hole  is  punched  in  the  lid  previous 
to  heating ;  this  is  left  open  until  the  steam  escapes  freely,  and  is 
then  sealed;  others  do  not  punch  this  hole  until  after  the  can  has 
been  heated  for  some  time,  and  still  others  do  not  consider  it  neces- 
sary to  vent  the  can  at  all.  The  advantage  of  venting  the  can  when 
it  is  first  placed  in  the  bath,  and  allowing  it  to  remain  open  until  the 
steam  issues  freely,  are  several.  In  the  first  place,  the  air  is  com- 
pletely removed  before  it  has  had  time  to  act  upon  the  fruit.  If 
sealed  first  and  vented  after  it  is  hot,  the  confined  air  tends  to  burst 
the  can,  and  also  acts  upon  the  flavoring  matter  of  the  vegetables,  and 
tends  to  make  it  somewhat  stale.  But  the  greatest  advantage  of 
venting  is  the  fact  that  a  can  vented  and  sealed  when  hot  partially 
collapses  as  it  cools,  and  the  heads  sink  in ;  so  long  as  the  can  is  in 
good  condition  the  heads  remain  concave,  but  if  by  any  accident  it 
commences  to  ferment  the  heads  at  once  swell  out,  and  it  becomes 
what  is  known  in  the  trade  as  a  "  swell  head." 
Meats,  from  their  greater  susceptibility  to  change  and  decay,  are 
much  harder  to  preserve,  and  recourse  is  generally  had  to  some  chem- 
ical agent.  The  most  common  of  these  are  the  alkaline  or  earthy- 
alkaline  sulphites.  These  are  added  in  a  minute  proportion  to  the 
contents  of  the  can  before  sealing,  and  serve  either  to  destroy  what 
little  free  oxygen  remains  in  the  can,  by  combining  with  it,  or,  what 
is  more  probable,  act  as  poisons  to  infusoria. 
Some  persons  have  conceived  the  idea  that  canned  goods  are  not 
healthy,  and  much  difficulty  has  been  experienced  in  introducing  can- 
ned Australian  and  South  American  meats  among  the  working  classes 
in  England,  from  their  being  prejudiced  against  articles  that  they 
could  not  inspect  before  buying.  This  prejudice  is,  we  are  happy  to 
say,  rapidly  dying  out.  The  State  Board  of  Health  of  Massachusetts 
made  some  elaborate  investigations  upon  the  subject  of  the  contami- 
nation of  such  articles  by  lead,  tin  or  copper  from  the  cans.  These 
were  a  complete  failure  so  far  as  proving  any  injurious  quantities  of 
these  metals  to  be  present. 
The  length  of  time  which  articles  thus  sealed  may  be  preserved  is 
still  unknown,  and  there  is  no  reason  to  doubt  that  they  will  be  kept 
perfectly  until  the  cans  are  destroyed  by  external  agencies.  Dr. 
32 
