Am.  Jour.  Pharm. 
Aug.,  1893. 
Determining  Fat  in  Milk, 
407 
To  this  mixture  are  added  50  cc.  of  a  light  petroleum  ether,  the 
specific  gravity  of  which  is  about  0-663,  the  boiling-point  6o°,  and 
which  evaporates  on  the  water-bath  without  residue.  The  glass  is 
stoppered  and  again  vigorously  shaken  so  as  to  form  an  emulsion. 
To  this  emulsion  are  added  50  cc.  alcohol  of  about  95-8  to  96  per 
cent.,  and  the  liquid  is  again  well  shaken.  After  at  most  four  or 
five  minutes  the  petroleum  ether  separates  at  the  top,  and  the  sepa- 
ration may  be  regarded  as  complete.  We  shake  again  three  or 
four  times,  each  time  for  a  quarter  of  a  minute,  allowing  each  time 
the  ether  to  separate  out. 
The  petroleum  ether  will  now  have  taken  up  all  the  fat.  We 
ascertain  this  point  by  shaking  up  eleven  specimens  a  different 
number  of  times,  the  first  once  and  the  eleventh  eleven  times. 
Already  after  the  third  or  fourth  shaking  we  have  found  quantities 
of  fat  which  differ  from  each  other  only  to  an  unimportant  degree. 
After  once  shaking  3-535  per  cent.,  after  twice  shaking  3*54  per 
cent.,  and  the  results  which  we  obtained  between  the  third  and 
eleventh  shaking  fluctuated  only  between  3-55  and  3-56  per  cent. 
Of  the  stratum  of  petroleum  ether,  20  cc.  are  drawn  off  with  a 
pipette  and  introduced  into  a  small  tared  capsule,  the  capacity  of 
which  is  about  40  to  50  cc,  and  the  neck  of  which  is  higher  than 
I  cm.,  with  a  diameter  of  1  to  2  cm.  These  small  flasks  are  con- 
venient, because  the  liquid  does  not  readily  rise  out  of  them,  and 
yet  the  evaporation  goes  on  with  sufficient  rapidity.  But  of  course 
small  tared  beakers  or  ordinary  flasks  may  be  used. 
The  flask  is  set  upon  a  water-bath  at  a  moderate  heat,  the  petro- 
leum ether  is  evaporated  entirely  away,  and  the  residue  is  dried  at 
from  no°  to  1200,  for  which  an  hour  is  generally  sufficient;  the 
weight  found,  if  multiplied  by  5,  gives  the  quantity  of  fat  in  ico  cc. 
The  quantities  of  fat  obtained  by  the  new  method  may  be  easily 
recalculated  by  the  aid  of  the  specific  gravity  into  percentages  by 
weight,  so  as  to  admit  of  a  comparison  with  the  Adams  method,  in 
which  the  milk  is  weighed.  We  remark  that  on  the  Adams 
method  the  extraction  with  petroleum  ether  must  last  for  at  least  3 
hours. 
The  results  of  the  new  method  vary  from  those  of  the  gravimetric 
method  by  0-066  in  a  positive  direction,  and  by  0  037  per  cent,  in 
a  negative  direction.  But  these  deviations,  in  our  opinion,  are  not 
necessarily  founded  on  the  sources  of  error  in  the  method,  but  are 
