366  Artificial  Salicylic  Acid.  {Am7u^Si?rm 
now  shown  by  their  physical  and  chemical  properties,  as  well  as  by 
the  analyses  of  their  silver  and  lead  salts,  to  be  orthocresotic  or 
orthohomosalicylic  acid[CH3 ;  OH  :  COOH  =  I  :  2  :  3],  and  metacre- 
sotic  or  metahomosalicylicacid[CH3:  OH:  COOH  ■  =  1:3:4]  respec- 
tively. The  ortho-acid  melts  at  1630  (corr.),  the  meta-acid  at  174-5° 
(coir.).  They  have  been  formed  from  the  corresponding  cresols 
contained  in  the  crude  phenol  from  which  the  salicylic  acid  had  been 
prepared. 
The  melting  point  of  pure  salicylic  acid  has  been  variously  stated, 
the  highest  recorded  temperature  being  1590  and  the  lowest  1 5  5 °. 
The  authors  find  that  "  natural  "  salicylic  acid,  after  recrystallization 
from  alcohol,  melts  at  156-75°  (corr.),  and  that  if  the  acid  melting 
at  this  temperature  be  converted  into  the  sodium  salt,  and  fraction- 
ally precipitated  with  silver  nitrate,  the  acid  recovered  from  each 
fraction  of  silver  salt  also  melts  at  156-75°,  which  may  therefore  be 
accepted  as  the  melting  point  of  the  pure  substance.  By  very 
slowly  cooling  a  I  percent,  solution  in  hot  water,  the  pure  acid  may 
readily  be  obtained  in  large,  distinct,  prismatic  crystals,  but  in  pres- 
ence of  about  5  per  cent,  of  one  of  the  cresotic  acids,  the  salicylic 
acid  no  longer  furnishes  large  crystals,  and  the  impure  acid  melts  at 
a  lower  temperature. 
A  specimen  of  the  artificial  salicylic  acid  of  commerce  (m.  p. 
154-5°)  was  examined  by  Williams'  method.  It  was  converted  into 
a  calcium  salt  by  boiling  with  water  and  calcium  carbonate,  and 
crystallized  many  times  from  water.  From  the  residue  of  soluble 
salt  from  which  most  of  the  calcium  salicylate  had  crystallized,  there 
was  obtained  a  small  quantity  of  an  acid,  which  after  further  purifi- 
cation melted  constantly  at  151°  (corr.).  Analysis  of  the  silver  and 
lead  salts,  as  well  as  its  physical  and  chemical  properties,  proved  it 
to  be  paracresotic  or  parahomosalicylic  acid 
(CH3  :  OH  :  COOH  =  1  :  4  :  3). 
For  the  purposes  of  preparing  pure  salicylic  acid  from  the  impure 
acid  of  commerce,  the  method  depending  on  the  different  solubilities 
of  the  calcium  salts  was  not  found  satisfactory,  being  extremely 
tedious,  owing  to  the  number  of  recrystallizations  that  are  necessary. 
A  better  plan  consists  in  preparing  the  lead  salts  by  the  action 
of  lead  carbonate,  and  crystallizing  these  from  dilute  alcohol ;  lead 
salicylate  being  much  less  soluble  than  the  lead  cresotates.  By 
