'2:1  Note  on  ^allcin.-  {"'^"jfu^'iSr""' 
I 
00°F.  being  taken  as  tlie  solubility ;  (2)  whether  the  substance  be 
added  in  excess  to  the  solv^ent,  the  mixture  being  filtered  and  a  known 
proportion  of  the  filtrate  evaporated  and  weighed ;  or,  (3)  whether 
the  substance  be  dissolved  in  various  proportions  by  heat,  and  the 
point  of  crystallization  observed  as  indicating  the  result  sought.  The 
first  method  is  most  reliable.  In  the  second  case  the  substance  (espe- 
cially if  organic)  may  not  be  in  the  same  condition,  when  dissolved 
and  evaporated  to  dryness,  as  ])efore  such  treatment ;  moreover  a  cor- 
rection is  necessary  for  the  increase  in  volume' on  addition  of  the  solid 
substance.  The  third  method  is  least  accurate,  as  a  supersaturated 
solution  may  remain  clear  ;  but  when  crystallization  has  commenced,, 
it  will  probably  continue  until  the  solution  is  much  weaker  than  a 
cold  saturated  one.  The  difference  of  a  few  degrees  of  temperature  is 
of  little  iinportiuice  except  wheu,  by  the  first  method  referred  to,  the 
observation  is  extended  over  a  long  period. 
In  Storer's  Ciiemical  Solubilities,"  various  observers  are  quoted 
giving  very  discordant  results,  viz.  :  "  One  part  of  salicin  dissolves  in 
17-  85  of  water  at  19  5°C. ;  in  14  at  17°C.;  in  30-31  at  11-5°C.;  in 
28'57  at  ordinary  temperatures ;  in  22  cold  and  -5  hot  water  ;  in  6  at 
18-  75°C. ;  in  5*6  cold  and  in  much  less  boiling  water."  The  two 
last  mentioned  may  be  misprints  for  16  and  15-6,  or  else  intended  for 
6  parts  and  5*6  respectively  soluble  in  100  parts  of  water. 
During  the  last  few  days  I  have  made  the  following  observations : 
Salicin  in  water  1  in  20,  finely  powdered  and  agitated  at  short 
intervals  for  twelve  hours  at  60°F.,  left  much  undissolved,  remaining 
so  for  several  days ;  1  in  20  dissolved  by  heat  and  filtered,  gave  a 
trace  of  crystals  in  five  hours  at  60°F.,  more  after  twelve  hours,  and 
a  large  increase  on  standing. 
Salicin  in  water  1  in  24,  cold  as  last  experiment,  left  less  undissolved 
after  twelve  hours  at  60°F.,  and  did  not  dissolve  entirely  after  several 
days;  the  same  dissolved  by  heat  and  filtered  gave  a  minute  trace  of 
crystals  after  six  hours  at  60°F.,  which  did  not  increase  on  standing. 
Salicin  in  water  1  in  28,  cold  as  before,  left  a  trace  undissolved  after 
twelve  hours  at  60°F.,  which  finally  disappeared ;  the  sauie  dissolved 
by  heat  showed  no  tendency  to  crystallize. 
Parallel  experiments,  using  a  10  per  cent,  aqueous  solution  of  gly- 
cerin instead  of  pure  water,  indicated  that  the  presence  of  this  body 
lessened  the  solubility  rather  than  otherwise,  1  of  salicin  in  24  giving 
a  large  crop  of  crystals  after  twenty-four  hours. 
