826  Lactometer  and  Fat  in  Milk  Control.  {Ami,Je°cT"i?2iarm' 
ponding  Quevenne  reading  will  be  29.  The  figure  for  the  specific 
gravity  thus  gives  at  a  glance  the  corresponding  Quevenne  figure. 
But  no  Mohr-Westphal  balance  outfit  should  be  depended  upon  until 
it  has  been  proved  to  meet  the  following  requirements:  (1)  the 
weights  of  the  riders  must  be  in  the  proportion  of  1  :  10  :  100  : 
1000;  (2)  the  divisions  on  the  beam  must  be  at  equal  intervals; 
and  (3)  the  outfit  must  show  in  water  of  some  known  tempera- 
ture the  density  water  is  known  to  possess  at  that  temperature.15 
If  a  Xew  York  Board  of  Health  lactometer16  is  used,  its  read- 
ing need  merely  be  multiplied  by  0.29  in  order  to  learn  the  corres- 
ponding Quevenne  reading,  for  ioo°  Board  of  Health  =  29°  Que- 
venne. 
In  any  event,  fluids  of  known  specific  gravity  should  be  avail- 
able for  calibrating  the  instrument  used.  In  dealing  with  this  mat- 
ter, a  most  important  point  to  be  borne  in  mind  is  that  while  most 
specific  gravities  are  referred  to  water  at  40  C,  lactometer  readings 
relate  to  gravities  at  60 0  F.  referred  to  water  at  6o°  F.17  When  the 
centigrade  scale  is  used,  the  density  Djjjcf;  is  usually  taken  as 
identical  with  the  density  D^o£\  The  writer  has  found  solutions 
of  sulphuric  acid  and  solutions  of  sugar  useful  for  such  calibra- 
tions. 
Sulphuric  acid  is  better  adapted  to  use  by  a  chemist  than  by 
others.  It  is  desirable  to  be  able  to  prepare  solutions  by  their  nor- 
malities, such  that  their  specific  gravities  will  correspond  to  certain 
points  on  the  lactometer  scale.  To  this  end  I  have  calculated  the  val- 
ues for  the  constants  in  the  following  approximation  formula  within 
the  range  of  specific  gravities  from  1.014  to  1.041 : 
10  Kohlrausch,  Tr.  Physical  Measurements,  1894,  p.  45. 
16  This  is  sometimes  called  the  Spence  scale.  "The  two  fixed  points  on 
the  scale  of  this  instrument  are  the  o°  mark  which  is  at  1.000  tne  specific  grav- 
ity of  pure  water,  and  the  ioo°  mark  which  is  set  at  1.029,  the  minimum  gravity 
of  milk.  The  graduations  are  continued  to  120°  and  1300.  The  point  1.029 
which  was  long  ago  fixed  in  Europe  as  the  limit  of  the  density  of  genuine  healthy 
milk  was  redetermined  in  1875  and  1876  by  the  health  authorities  of  New  York 
and  New  Jersey,  from  actual  experiments  at  the  dairies.  Out  of  1600  cows 
whose  milk  was  examined,  only  six,  two  of  whom  were  sick  at  the  time,  were 
found  to  give  milk  below  that  figure."  Pellew,  Man.  of  Prac.  Med.  and 
Physiol.  Chem.,  1893,  p.  178.  Every  degree  below  ioo°  was  supposed  to  repre- 
sent one  per  cent,  of  added  water.  No  such  interpretation  of  the  reading  of 
the  Spence  scale  is  today  acceptable. 
17  The  correction  for  temperatures  other  than  60°  F.  may  be  gotten 
from  any  book  dealing  with  milk  analysis;  for  example,  Leffmann's  Milk 
Analysis,  cited  in  fotnote  5. 
