Am.  Jour.  Pharm. 
July,  1891. 
Natal  Aloes. 
355 
.Now  take  the  case  of  sugar  in  solution.  The  molecular  weight 
expressed  by  the  formula  C12H22On  is  342.  In  a  1  per  cent,  sugar 
solution  therefore  the  molecular  weight  expressed  in  grams  occupies 
34,200  cc.  At  o°  C.  the  pressure  on  the  solvent  is  found  to  be 
equal  to  493  mm.  of  mercury,  that  is  to  say,  671  grams  per  square 
centimeter.    The  constant  for  sugar  is  therefore  : 
6^  x  34.200  =  84>200 
273 
which  agrees  to  within  6  parts  in  1,000  with  that  for  gases.  As 
this  relation  is  not  special  to  the  case  of  sugar  but  is  a  general 
relation,  it  follows  that  Avogadro's  law  is  obeyed  by  substances  in 
solution,  equal  volumes  of  which,  at  the  same  temperature,  and 
exercising  the  same  pressure  on  the  solvent,  contain  equal  numbers 
of  molecules. 
It  is  well  known  that  the  abnormal  vapor  densities  of  certain 
bodies  which  we  now  know  to  be  dissociated  in  the  gaseous  state 
long  opposed  an  obstacle  to  the  acceptance  of  Avogadro's  generali- 
zation. The  exceptions  met  with  in  the  case  of  substances  in  solu- 
tion are  of  exactly  parallel  character.  The  deviations  from  the 
law  are  always  in  one  direction,  giving  too  great  a  volume  when 
the  calculation  is  based  on  the  received  molecular  weights.  This 
is  met  by  the  hypothesis  that  such  exceptional  substances  are  to 
some  extent  dissociated  in  solution.  As  in  the  case  of  gases  so 
also  in  the  case  of  substances  in  solution,  many  bodies  occupy  a 
volume  twice  as  great  as  that  calculated  from  their  molecular 
weight.  These  are  regarded  as  being  completely  dissociated  into 
two  constituents.  The  most  important  classes  of  substances  which 
behave  thus  are  the  strong  acids  and  bases  and  their  salts.  A  large 
number  of  phenomena  afford  evidence  in  support  of  this  hypothesis 
of  dissociation  in  solution,  the  study  of  which  now  constitutes  an 
important  field  of  work  connected  with  the  development  of  the  new 
theory  of  solution. — Phar.  Jour,  and  Trans.,  May  16,  p.  1018. 
NATAL  ALOES. 
By  E.  M.  Holmes,  F.L.S. 
There  has  for  many  years  been  a  difficulty  in  determining  the 
botanical  source  of  the  article  known  in  commerce  as  "  Natal,"  or 
sometimes  as  "  Hepatic  "  Cape  aloes. 
In  appearance  the  drug  bears  a  strong  resemblance  in  color  and 
