4°  Proteids.  { Am  /a°nu,ri8^rm- 
their  solutions  are  not  coagulated  by  heat ;  they  are  non  diffusible 
and  are  precipitated  by  saturating  their  solutions  with  ammonium 
sulphate.  The  last  two  characters  distinguish  them  from  peptones, 
to  which  they  are  nearly  related.  They  are  chiefly  met  with  as  inter- 
mediate products  in  gastric  and  tryptic  digestion.  Three  varieties 
have  been  described  :  (i)  Proto- albumose ;  (2)  hetero-albumose  ;  (3) 
deutero-albumose. 
Albumose  also  occurs  in  snake  poison.  In  diphtheritic  membrane 
an  albumose  possessing  a  powerful  toxic  action  is  formed. 
Vegetable  Albumoses. — Jequirity  seed  contains  a  poisonous  albu- 
mose as  well  as  the  globulin  before  mentioned.  Albumoses  also 
occur  as  constituents  of  aleurone  grains. 
Class  5,  Peptones. — These  are  the  final  products  of  the  gastric 
digestion  of  proteids.  Their  solutions  are  not  coagulated  by  heat. 
They  are  diffusible  and  are  not  precipitated  by  saturation  with 
ammonium  sulphate.  This  last  property  is  exceedingly  valuable* 
enabling  us  to  separate  peptones  from  other  proteids,  all  the  rest, 
albumoses  included,  being  precipitated  by  saturating  their  solutions 
with  this  salt. 
Two  kinds  of  peptone  are  formed  by  gastric  digestion  (1)  hemi- 
peptone  and  (2)  anti-peptone.  Pancreatic  digestion  has  no  further 
action  on  anti-peptone  while  hemi-peptone  is  further  split  up  into 
simpler  products  which  are  no  longer  proteids,  and  of  which  leucin 
and  tyrosin  are  the  chief.  Anti-peptone  does  not  give  Millon's 
reaction ;  the  significance  of  this  will  be  seen  later  on. 
Class  6,  Coagulated  Proteids. — This  term  is  applied  to  the  coagula 
produced  by  the  action  of  heat  on  solutions  of  albumins  and  globu- 
lins. Hard-boiled  white  of  egg  is  a  familiar  example.  They  are 
insoluble  in  water  and  saline  solutions;  in  acids  and  alkalies  they  are 
partially  soluble,  but  only  by  long-continued  digestion. 
Constitution  of  Proteids. — Very  little  is  known  about  the  constitu- 
tion of  proteid  molecules,  except  that  they  are  exceedingly  complex 
and  liable  to  change  under  comparatively  slight  external  influences. 
Probably  peptones  have  the  simplest  constitution.  They  are  derived 
from  all  the  other  proteids  by  the  action  of  ferments — an  action 
always  accompanied  by  hydrolysis — and  from  analogy  with  the  sugars 
derived  by  a  similar  process  from  the  more  complex  starch  molecule 
it  may  be  supposed  that  they  have  a  simpler  constitution  and  smaller 
molecular  weight  than  the  other  proteids,  from  which  they  are 
