AmMay?i9(X)arm'}    Classification  of  Odoriferous  Principles.  221 
So  much  has  been  added  to  our  knowledge  of  the  essential  oils 
in  recent  years,  and  so  much  has  been  done  in  the  line  of  synthetic 
work  in  this  domain,  that  a  survey  of  the  whole  material  from  the 
chemical  standpoint  is  very  welcome. 
The  author  points  out  that  we  must  first  of  all  concede  that  if 
a  substance  is  to  act  upon  the  olfactory  nerves  it  must  be  volatile. 
But  all  gases  do  not  act  upon  the  sense  of  smell,  and  so  we  must 
seek  for  a  reason  for  the  activity.  He  is  inclined  to  believe  that  it 
resides  in  the  development  of  a  true  chemical  reaction  between  the 
odoriferous  volatile  substance  and  the  protoplasmic  matter  of  the 
cells  forming  the  olfactory  nerves.  Thus  the  action  of  aldehydes, 
to  which  class  a  large  number  of  odoriferous  principles  belong, 
upon  protein  substances  has  been  noted  and  recently  made  the 
basis  of  a  patent  application  by  the  Elberfeld  Farbenfabrik  Co.  When 
two  substances  of  different  chemical  constitution  like  nitrobenzene 
and  benzaldehyde  seem  to  possess  the  same  odor,  we  have  an 
anomaly,  but  we  have  similar  anomalies  in  the  similarity  of  taste  of 
cane  sugar  and  saccharine. 
The  author  calls  attention,  in  considering  the  action  of  a  perfume, 
to  the  difference  between  quality  of  odor  and  intensity  or  pene- 
trating power  of  the  same.  The  first  he  considers  to  be  absolutely 
dependent  upon  the  chemical  constitution  of  the  perfume;  the 
second,  while  somewhat  dependent  upon  chemical  nature,  is  rather 
connected  with  its  physical  properties,  such  as  volatility  and  special 
conditions  of  admixture  with  air  or  vapors  of  other  volatile  bodies 
such  as,  for  instance,  alcohol. 
He  divides  the  distinctive  odoriferous  principles  into  seven  main 
groups,  viz.:  (1)  aldehydes;  (2)  alcohols  and  esters;  (3)  ketones; 
(4)  phenols  and  phenol  ethers;  (5)  acid  and  acid  anhydrides;  (6) 
nitrogenous  substances  ;  (7)  hydrocarbons. 
The  first  and  most  important  class  are  the  aldehydes.  The  lowest 
members  of  this  class,  the  aldehydes  of  the  fatty  series,  like  formal- 
dehyde, acetaldehyde,  butyraldehyde,  valeraldehyde,  are,  it  is  true, 
found  at  times  in  essential  oils,  but  the  author  does  not  include 
them  in  his  list,  as  they  are  irritating  and  unpleasant  in  odor,  con- 
tributing in  no  way  to  the  value  of  a  perfume. 
This  group  includes,  as  is  seen,  several  interesting  substances  of 
artificial  or  synthetic  manufacture,  such  as  citral,  benzaldehyde* 
cinnamic  aldehyde,  vanillin  and  piperonal. 
