AmApTi];iP907arm'}  Chemistry  of  the  Proteins.  169 
differs  widely.  The  nature  of  fats  was  essentially  proved  by  the 
researches  of  Chevreul  on  the  preparations  of  soaps  in  the  first 
decade  of  the  nineteenth  century;  and  in  1854,  or  twenty-six  years 
after  the  commencement  of  organic  synthesis,  Berthelot  showed  that 
it  was  possible  to  prepare  the  fats  from  glycerol  and  the  fatty  acids. 
Whereas  it  was  not  until  1890  that  grape  sugar — the  most  impor- 
tant member  of  the  carbohydrate  group — was  artificially  prepared, 
and  the  more  complicated  members — starch  and  cellulose — have 
not  even  now  been  synthesized,  and,  moreover,  the  size  of  their 
molecules  is  doubtful.  Still  more  imperfect  is  our  knowledge  of 
the  proteins  (albuminoids),  which  differ  from  the  fats  and  carbo- 
hydrates in  containing  nitrogen,  and  are,  together  with  their 
numerous  derivatives,  the  most  complicated  chemical  substances 
which  Nature  produces.  Typical  of  them  is  casein — obtainable 
in  a  variety  of  ways  from  milk  and  the  white  of  egg — which  is  not, 
as  generally  supposed,  an  individual  substance,  but  consists  of  at  least 
two  proteins  resembling  one  another  very  closely.  The  property 
possessed  by  egg  albumin  of  coagulating  when  heated  is  common 
to  a  large  number  of  proteins.  Blood  is,  however,  richer  in  these 
substances  than  other  animal  secretions,  containing  four  different 
classes,  to  which  belong  fibrin  and  globin.  Two  other  proteins 
worthy  of  special  mention  on  account  of  their  simple  chemical  com- 
position are  (a)  the  protamines — of  which  the  first  representative 
was  isolated  by  Miescher  in  1874  from  salmon  spawn — and  (^) 
fibroin,  the  principal  constituent  of  silk,  which,  according  to  the 
author's  experience,  is  the  easiest  of  all  proteins  to  study. 
STRUCTURE  OF  THE  PROTEINS. 
As  in  the  other  chapters  of  organic  chemistry,  two  methods  are 
available  for  determining  the  structure  of  the  molecules  of  the  pro- 
teins— synthesis  and  analysis — and  in  connection  with  the  latter,  the 
only  process  which  has  given  direct  evidence  is  "  hydrolysis," 
whereby  there  are  first  formed  easily  soluble  products  known  as 
albumoses  and  peptones,  which  then  further  break  down  into  amino 
acids.  Such  changes  occur  during  digestion,  and  can  be  more 
rapidly  brought  about,  artificially,  by  the  action  of  strong  acids, 
such  as  hydrochloric  acid,  which  gives  rise  almost  exclusively  to 
amino  acids.  Among  the  more  important  of  these  may  be  men- 
tioned glycocoll,  alanine,  valine,  leucine,  and  isoleucine,  which  are 
