38 
Proteids. 
<  Am.  Jour.  Pharm. 
I        Jan..  1893. 
proteids.  Separate  the  yolk  of  the  egg  carefully  and  place  the  white 
in  a  dish ;  cut  it  with  scissors  in  all  directions,  in  order  to  liberate 
it  from  the  membranes  in  which  it  is  enclosed.  Shake  up  the  fluid 
with  about  twenty  times  its  volume  of  water,  and  to  the  solution  so 
obtained,  after  separation  of  the  membranous  fragments,  apply  the 
following  tests  : 
(1)  Xanthoproteic  Reaction. — Add  strong  nitric  acid  ;  a  precipitate 
is  formed  which  turns  yellow  on  boiling;  cool  and  add  ammonia; 
the  precipitate  turns  to  an  orange  color. 
(2)  Biuret  Reaction. — Add  a  little  very  dilute  solution  of  copper 
sulphate  and  then  sodium  hydrate ;  a  violet  color  is  produced 
(certain  proteids — the  albumoses  and  peptones — give  a  rose-red 
color). 
(3)  Add  Millon's  reagent  (solution  of  mercuric  nitrate  in  nitric 
acid).     A  white  precipitate  falls  which  becomes  reddish  on  boiling. 
(4)  To  demonstrate  the  presence  of  nitrogen  (i)  heat  some  dry 
proteid  in  a  test  tube  and  suspend  in  the  tube  a  piece  of  red  litmus 
paper.  Ammonia  is  formed  by  the  destructive  action  of  heat  and 
changes  the  color  of  the  red  litmus  to  blue  ;  (ii)  heat  the  dry  proteid 
with  a  piece  of  metallic  sodium.  Sodium  cyanide  is  formed  and 
may  be  detected  in  the  usual  way  by  means  of  a  ferrous  and  a  ferric 
salt. 
(5)  To  demonstrate  the  presence  of  sulphur  (i)  repeat  test  4  (i), 
but  replace  the  litmus  by  a  piece  of  lead  acetate  paper.  It  blackens 
owing  to  the  formation  of  sulphuretted  hydrogen  ;  (ii)  heat  with 
caustic  soda  solution  and  lead,  acetate.  The  mixture  darkens, 
becoming  dark  brown  or  black  owing  to  the  liberation  of  sulphu- 
retted hydrogen  (by  the  action  of  the  caustic  alkali  on  the  proteid) 
and  consequent  formation  of  lead  sulphide. 
Classification  of  Proteids. — The  chief  points  assisting  in  the  dis- 
tinction and  classification  of  proteids  are  their  solubility  or  insolu- 
bility in  (a)  water,  (p)  saline  solutions  of  various  strengths,  (c)  acids 
and  alkalies  and  (d)  the  temperature  of  coagulation. 
Class  /,  Albumins. — These  are  soluble  in  distilled  water,  their 
solutions  being  coagulated  at  from  65— 730  C.  The  following  are 
members  of  this  class  : 
Egg  Albumin. — White  of  egg  contains  several  varieties  of  albumin, 
which  may  be  separated  by  fractional  heat  coagulation. 
Serum  Albumin. — Blood  serum  also  contains  several  nearly  allied 
