510  ABSORPTION  AND  DIALYTIC  SEPARATION  OF  GASES,  ETC. 
floating  in  the  first  tank  are  rejected  ;  those  which  float  in  the 
second  tank  form  the  third  quality  of  beet ;  those  which  float  in 
the  third  tank  constitute  the  second  quality  of  beet,  and  those 
which  sink  m  it  make  the  first  class  of  beet.  The  cost  of  the  pro- 
duction of  beet-sugar  has  been  reduced  to  about  four  cents  per 
pound  ;  the  tax  is  about  three  cents,  other  charges  about  1J  cent, 
making  the  selling  price  from  9  to  11  cents  per  pound. — Drug- 
gists Circular ',  Sept.,  1866,  from  Sorgho  Journal. 
ON   THE  ABSORPTION  AND  DIALYTIC  SEPARATION  OF 
GASES  BY  COLLOID  SEPTA. 
By  Thomas  Graham,  F.R.S. 
It  appears  that  a  thin  film  of  caoutchouc,  such  as  is  furnished 
by  varnished  silk  or  the  transparent  little  balloons  of  india-rub- 
ber, has  no  porosity,  and  is  really  impervious  to  air  as  gas.  But 
the  same  film  is  capable  of  liquefying  the  individual  gases  of 
which  air  is  composed,  while  oxygen  and  nitrogen  in  the  liquid 
form  are  capable  of  penetrating  the  substance  of  the  membrane 
(as  ether  or  naphtha  does),  and  may  again  evaporate  into  a 
vacuum  and  appear  as  gases.  This  penetrating  power  of  air  be- 
comes more  interesting  from  the  fact  that  the  gases  are  unequally 
absorbed  and  condensed  by  rubber,  oxygen  2J-  times  more 
abundantly  than  nitrogen,  and  that  they  penetrate  the  rubber  in 
the  same  proportion.  Hence  the  rubber  film  may  be  used  as  a 
dialytic  sieve  for  atmospheric  air,  and  allows  very  constantly 
41  "6  per  cent,  of  oxygen  to  pass  through,  instead  of  the  21  per 
cent,  usually  present  in  air.  The  septum  keeps  back,  in  fact, 
one-half  of  the  nitrogen,  and  allows  the  other  half  to  pass  through 
with  all  the  oxygen.  This  dialysed  air  rekindles  wood  burning 
without  flame,  and  is,  in  fact,  exactly  intermediate  between  air 
and  pure  oxygen  gas  in  relation  to  combustion. 
One  side  of  the  rubber  film  must  be  freely  exposed  to  the  at- 
mosphere, and  the  other  side  be  under  the  influence  of  a  vacuum 
at  the  same  time.  The  vacuum  may  be  established  within  a  bag 
of  varnished  silk  or  in  a  little  balloon,  the  sides  being  prevented 
from  collapsing  by  interposing  a  thickness  of  felted  carpeting 
between  the  sides  of  the  varnished  cloth,  and  by  filling  the 
