Am-jan.r;if9h3arm'}      Recent  Work  in  the  Sugar  Group.  33 
The  reagent  which  has  proved  more  useful  than  any  other  in  the 
study  of  the  sugars  is  phenylhydrazine,  CGH5HNNH2.  The  com- 
pounds which  it  forms  with  glucose  may  be  taken  as  representative 
of  the  group  ;  on  mixing  solution  of  phenylhydrazine  acetate  with 
solution  of  glucose,  water  is  eliminated  and  the  residue  of  the 
hydrazine  replaces  the  oxygen  atom  of  the  aldehyde  group,  thus — 
CH2OH(CHOH),CH(N-NHC6H5). 
On  heating,  more  phenylhydrazine  enters  into  the  reaction,  first 
removing  two  atoms  of  hydrogen  from  the  next  carbon  atom,  and 
then  replacing  the  oxygen  attached  to  this  carbon  in  a  manner 
similar  to  the  first  action  ;  this  gives  a  product  of  the  formula — 
CH2OH(CHOH)8C(N-NHC6H5)CH(N-NHC6H5). 
The  form  of  these  compounds,  containing  one  hydrazine  residue,  is 
called  a  phenylhydrazone,  and  the  latter,  containing  two  such 
residues,  a  phenylosazone  ;  since,  however,  the  hydrazine  employed 
is  always  the  phenyl  compound,  the  prefix  phenyl-  is  commonly 
dropped,  and  the  substances  are  referred  to  as  hydrazones  and 
osazones,  respectively.  The  hydrazones  are,  as  a  rule,  soluble  com- 
pounds ;  the  osazones  are  but  very  slightly  soluble,  and,  being 
crystalline  substances  of  definite  melting  point,  are  very  valuable 
for  characterizing  and  identifying  the  sugars.  A  sugar  can  be  easily 
regenerated  from  either  its  hydrazone  or  osazone. 
Optically  isomeric  sugars  are  distinguished  by  the  prefixed  letters 
d-,  1-  and  i-.  The  letter  d-  is  used  to  denote  those  sugars  which  occur 
in  nature,  as  most  of  them  are  dextro-rotatory,  and  those  which  are 
laevo- rotatory,  as  ordinary  fructose,  are  shown  by  their  behavior  to 
belong  to  the  same  series.  The  optical  opposites  of  the  d-  sugars 
are  indicated  by  the  letter  1-,  and  the  inactive  substances  resulting 
from  mixing  the  d-  and  1-  compounds,  by  the  letter  i-.  Alcohols, 
acids,  and  other  compounds  have  the  prefix  which  belongs  to  the 
sugar  from  which  they  are  obtained,  without  regard  to  their  own 
activity. 
Perhaps  the  most  important  branch  of  this  work  is  that  which 
relates  to  the  artificial  preparation  of  sugars  and  allied  compounds 
which  have  previously  been  obtained  only  from  natural  sources.  If 
dibromacraldehyde  is  treated  with  baryta,  condensation  of  the 
former  occurs  with  elimination  of  bromine,  and  a  6-carbon  sugar  is 
formed  ;  on  conversion  into  the  osazone  and  fractional  crystallization 
