40  Physiological  Action  of  Salicylic  Acid.  {Ami£Z'mum' 
which  one  mode  of  purifying  the  artificial  salicylic  acid  was  given. 
Proceeding  on  the  lines  Mr.  Williams  laid  down,  and  modifying  in 
the  final  part  his  process  by  employing  repeated  slow  crystallization 
from  water  instead  of  rectified  spirit,  a  purified  artificial  salicylic 
acid  was  obtained  identical  in  its  chemical  form,  its  solubility,  its 
melting-point  and  its  physiological  action  with  that  of  the  natural 
acid.  It  was  also  shown  that  if  the  form,  the  solubility  and  the 
melting-point  varied  in  the  artificial  from  that  of  the  natural,  no 
uniform  physiological  result  could  be  obtained. 
I  did  not  venture  to  give  a  name  to  the  impurity  or  impurities 
which  could  be  extracted  from  commercial  salicylic  acid.  I  only 
said  they  were  probably  some  form  of  cresotic  acid,  and  left  their 
investigation  to  chemists  able  and  willing  to  make  an  exact  analysis. 
On  the  4th  of  October  I  received  with  great  pleasure  a  letter 
from  Professor  Dunstan,  whose  capabilities  as  a  careful  and'  com- 
petent chemist  are  well  known  in  scientific  circles,  stating  that  he 
had  made  commercial  salicylic  acid  a  matter  of  investigation,  and 
asking  my  co-operation  in  regard  to  the  physiological  action  of  the 
constituents  which  he  had  found  in  it.  My  assent  was  willingly 
given,  and  the  results  of  these  experiments  will  now  be  detailed. 
On  the  7th  of  October,  1890,  I  received  from  Professor  Dunstan 
a  sample  of  artificial  salicylic  acid  and  two  samples  of  substances 
which  had  been  removed  from  this  acid  and  marked  O-acid  and  P-acid, 
and  I  subsequently  received  another  substance  marked  M-acid.  For 
convenience  of  description  the  characters  of  these  samples  will  be 
given  in  their  alphabetical  order,  although  the  details  of  their 
physiological  action  are  recorded  in  the  order  of  time  in  which  they 
were  examined. 
M-acid  (meta-cresotic  acid). — Hard  white  crystals,  melting-point 
•  174-5°  C. 
O-acid  (ortho-cresotic  acid). — White  silky  crystals,  very  light, 
melting-point  1630  C.  - 
P-acid  (para-cresotic  acid). — Whitish  powder,  somewhat  like 
coarse  flour,  melting-point  15 1°  C. 
Artificial  salicylic  acid. — Small  crystals  with  slight  interlacing 
needles,  melting-point  1 5 70  C. 
These  samples  were  examined  as  to  their  physiological  action  in 
the  following  way. 
O-acid.    October  18,  1890 — 10  a.m. — Two  grains  of  this  sample 
