The  sputum  now  homogeneous,  it  remains  to  concentrate  the
bacilli.  This  cannot  l>e  done  by  simple  centrifugalization.  since  the
specific  gravity  of  the  bacilli  is  the  same  or  lighter  than  that  of  the
fluid,  and  after  standing  even  eight  times  as  many  may  be  found  in  the
supernatant  fluid  as  in  the  sediment.  The  specific  gravity  of  the  fluid
must  be  changed,  made  lower  than  loio  or  higher  than  1080  {the

4


50  CLINICAL   DIAGNOSIS

limits  of  the  specific  gravity  of  a  culture  of  these  bacilli).  The  addi-
tion of  an  equal  amount  of  alcohol  will  accomplish  the  fomier,  but
alcohol  seems  to  affect  the  bacilli;  or  an  equal  volume  of  25  per  cent.
NaCl  solution  added  and  the  bacilli  removed  later  from  the  upper
layers.  The  specific  gravity  of  sputum  varies  from  0.929  to  1.2242.
It  is  well  to  add  a  little  egg  albumin  to  fix  the  organisms  to  the  glass.
Some  prefer  to  make  the  specimens  on  cover-glasses;  others  use  slides,
which  is  easier  and  allows  a  larger  surface.  The  smears  are  best  made
with  a  needle,  holding  the  slide  above  a  flame  at  such  a  distance  that
the  specimen  is  merely  warmed.    The  mucus  then  is  easily  spread.
Stahiing. — The  tubercle  bacillus  is  acid-fast,  but  as  there  are  other
organisms  with  this  property  to  the  same  or  a  less  degree,  one  must
be  on  his  guard.  The  Ziehl-Xeelsen  carbol  fuchsin  is  generally  used
(fuchsin,  1  gni. ;  absolute  alcohol.  10  cc. ;  5  per  cent,  carbolic  acid
100  cc. ).  The  specimen  may  be  stained  either  by  heat  or  in  the  cold.
