TINFOIL  AS  A  PRESERVATIVE,  ETC. 
435 
stances  liable  to  change  rs  still  rather  limited,  and  there  seemed 
to  be  a  prospect  of  its  admitting  of  a  more  general  use  than  has 
hitherto  been  made  of  it.  At  the  same  time  there  was  an 
absence  of  any  precise  experiments  for  the  purpose  of  deter- 
mining in  a  scientific  manner  the  degree  of  impenetrability  of 
tinfoil.  Having  been  engaged  for  some  time  in  the  investigation 
of  this  subject,  I  have  obtained  the  following  results  : — 
For  many  years  past  I  have  observed  that  cacao  butter,  which 
readily  becomes  rancid  even  when  kept  in  bottles  into  which  it 
has  been  introduced  in  the  melted  state,  if  the  bottles  be  opened 
from  time  to  time,  does  not  undergo  the  same  change  when 
moulded  in  tablets  and  wrapped  in  tinfoil.  This  fact,  which  was 
confirmed  by  many  observations,  and  could  only  be  explained  by 
assuming  the  impenetrability  of  tinfoil  to  atmospheric  air,  formed 
the  starting-point  for  some  experiments  in  the  same  direction, 
which  proved  satisfactory.  Thus,  a  piece  of  well-burned  quick- 
lime, enclosed  in  a  double  wrapper  of  tinfoil,  was  exposed  in  the 
atmosphere  of  the  laboratory  by  the  side  of  another  similar  piece 
which  was  exposed  without  protection.  While  the  latter  became 
slacked,  that  which  was  protected  by  the  tinfoil,  and  weighed 
92*2  grams  on  the  1st  of  December,  1867,  had  only  gained  3 
decigrams  in  weight  at  the  expiration  of  a  month,  and  after  being 
kept  until  the  25th  of  March,  1868,  it  had  only  increased  to  94 
grams.  It  had  thus  gained  only  1*8  grams  in  four  months.  On 
being  then  taken  out  of  its  metallic  envelope  much  heat  was 
developed  from  absorption  of  moisture,  and  it  fell  into  powder. 
Satisfied  by  this  experiment  of  the  efficacy  of  tinfoil  for  pre- 
serving bodies  from  the  action  of  air  and  moisture,  it  seemed 
probable  that  substances  the  most  susceptible  of  change  might  be 
kept  in  the  same  way.  It  was  found  that  substances  so  deliques- 
cent as  chloride  of  calcium  and  liver  of  sulphur,  and  efflorescent 
salts  such  as  carbonate  and  sulphate  of  soda,  remained  almost 
unchanged  when  wrapped  in  tinfoil,  increasing  or  diminishing 
only  to  a  few  thousandths  of  their  weight  in  several  weeks. 
Other  experiments  were  made  of  a  more  precise  character.  It 
is  well  known  that  fresh  lemons  become  rapidly  dried  and 
ultimately  hard  when  exposed  to  the  air,  and  that  they  also  be- 
come perished  and  covered  with  mould.    I  had  endeavored  to 
