Am-Ja°n.?1891rm-}  Proteids.  ^ 
derived.  Moreover,  any  further  change  produced  by  chemical  or 
fermentative  action  on  peptones  results  in  the  formation  of  simpler 
bodies,  mostly  of  known  composition,  which  are  no  longer  proteids. 
The  chief  obstacle  in  the  way  of  a  correct  determination  of  proteid 
molecular  weight  is  the  difficulty  of  obtaining  a  quantity  sufficient 
for  analysis  in  an  absolutely  pure  condition.  They  are  so  liable  to 
change  that  the  ordinary  methods  of  determining  molecular  weight 
are  not  applicable.  Attempts  have  been  made  to  determine  their 
molecular  weight  by  the  analysis  of  compounds  with  various  sub- 
stances. The  facility  with  which  proteids  combine  with  these  sub- 
stances in  various  proportions  tends  to  discredit  the  belief  that  they 
are  true  chemical  compounds.  For  instance,  the  formula  C20iH322N52 
066S2has  been  proposed  for  egg  albumin  from  the  analysis  of  a  copper 
compound  formed  when  copper  solutions  are  added  to  neutral  solu- 
tions of  the  albumin.  From  the  relative  quantities  of  sulphur  and 
iron  contained  in  carefully  purified  crystals  of  haemoglobin  from 
horses'  blood  the  formula  C68oH109SN210S2Om  has  been  calculated  for 
the  proteid  constituent  giobin.  Compounds  of  magnesium,  sodium 
and  calcium  with  the  globulins  from  the  proteid  crystals  of  seeds 
have  also  been  analyzed  and  the  formula  C292H481N90O83S2  derived 
therefrom.  Whether  or  not  any  of  these  formulae  are  true  or  near 
the  truth  it  is  certain  that  the  proteid  molecule  is  large  and  compli- 
cated. This  is  shown  by  the  great  number  and  variety  of  decompo- 
sition products  which  proteids  yield  when  subjected  to  various 
reagents.  In  addition  to  simpler  bodies  the  following  may  be  men- 
tioned :  glycocoll,  leucin,  aspartic  acid,  glutamic  acid  and  tyrosin. 
By  putrefactive  changes  indol  and  skatol  are  also  produced.  The 
general  proteid  reactions  are  probably  due  to  various  bodies  pro- 
duced or  liberated  by  the  action  of  the  reagents  on  proteids.  Thus 
the  biuret  reaction  is  so-called  because  the  substance  biuret 
yields  a  very  similar  c^ior.  Biuret  may  be  formed  by  heating 
urea — 
NH2      MU  CO<Ai 
C0<NH, 
2CO<NH;-NH-m3NH 
Cyanogen  compounds  are  formed  by  further  heating  biuret,  cyanuric 
acid  being  one.  These  also  give  the  biuret  reaction.  Hydrocyanic 
acid,  however,  gives  the  pink  color  like  peptones  and  albumoses. 
From  this  it  has  been  inferred  that  proteids  contain  a  cyanogen 
