390 
Lactic  Acid. 
(  Am.  Jour.  Phakm. 
1    Sept.  1, 1873. 
The  spec.  grav.  of  pure  monohydrated  lactic  acid  is  1*245  at  20° 
C,  or  1-248  at  15°  C.  It  is  very  frequently  quoted  in  text-books 
erroneously  as  having  a  spec.  grav.  of  1  215  at  20°  C.  (/.  *.  Kekule, 
Lehrbuch  der  organischen  Chemie,  i,  p.  748.  Fownes,  Am.  ed.  1870, 
p.  646,  etc.)  The  relative  strength  of  the  pure  acid  and  those  of  the 
German  and  U.  S.  Pharmacopoeias  may  be  seen  in  the  following  table  : 
1  eq.  or  90  parts  of 
HO.CJIA 
Germ.  Ph.  acid. 
U.  S.  Ph.  acid. 
Saturate  of  KO,HO,2C02. 
100-1  (K  =  39-l) 
97-97 
75 
Per  cent,  of  real  acid. 
100  per  cent. 
97 
74-88  " 
The  samples  which  I  examined  were  the  following  : 
1.  Merck's  acid,  bought  1872,  of  a  light  yellow  color,  is  a  little 
ropy,  has  scarcely  any  odor,  and  is  free  from  traceable  impurities. 
2.  Merck's  acid,  bought  1873,  of  a  faint  yellow  color,  is  very 
clear,  almost  odorless  and  pure. 
3.  Trommsdorf's  acid  (1873)  is  perfectly  colorless,  brilliant,  of  a 
faint  ethereal  odor,  and  pure. 
4.  Gehe  &  Co.'s  acid  (1873)  is  of  a  straw  color,  and  has  a  slight 
butyraceous  smell  ;  otherwise  pure. 
5.  Marquart's  acid  (1870)  is  quite  yellow,  rather  thin,  has  consid- 
erable odor,  but  is  otherwise  apparently  pure. 
6.  Marquart's  acid  (1871)  is  faintly  yellow,  a  little  thicker  than 
the  other  and  almost  odorless  ;  pure. 
The  assay  was  made  volumetrically  by  standard  solution  of  soda, 
in  each  case  upon  three  separate  weighed  portions,  and  the  figures 
given  below  are  the  mean  of  three  assays.  And  here  1  would  remark 
that,  in  weighing  the  lactic  acid  for  the  purpose  of  assay  or  analysis, 
or  whenever  great  accuracy  is  required,  it  is  absolutely  necessary  to 
guard  against  its  abstracting  any  moisture  from  the  atmosphere.  In 
this  respect  it  is  as  hygroscopic  as  sulphuric  acid.  It  should  always 
be  weighed  by  the  method  of  "  subtraction."  If  a  small  portion  is 
required  to  be  weighed,  introduce  a  corresponding  quantity  into  a 
vial  provided  with  a  well  ground  stopper,  taking  care  not  to  get  any 
on  the  neck  or  rim,  and  weigh  ;  pour  out  a  quantity  deemed  to  be 
sufficient,  replace  the  stopper  immediately,  and  slip  over  the  neck  a 
rubber  cap,  which  was  previously  weighed,  together  with  the  vial  and 
acid  (an  unperforated,  well-cleaned  rubber  nipple  answers  well),  and 
