54 
Titer  mo  meters. 
( Am.  Jour.  Pharm. 
\    February,  1904. 
with  it  all  traces  of  air  and  moisture.  On  cooling,  the  mercury  is 
drawn  down  into  the  bulb  which  it  completely  fills.  After  standing 
a  while,  should  any  air  bubbles  appear,  the  operation  has  to  be 
repeated,  for  the  presence  of  air,  even  in  minute  quantities,  is  sure 
to  work  mischief.  When  the  thermometer  is  properly  filled  the 
upper  bulb  is  cut  off  and  the  tube  drawn  out  to  a  fine  point.  It  is 
then  placed  in  a  liquid  heated  to  the  temperature  that  its  extreme 
height  is  to  represent,  and  after  all  excess  of  mercury  has  escaped  it 
is  sealed  over  in  the  flame,  and  this  completes  the  thermometer.  It 
has  been  observed  that  after  being  laid  aside  for  a  time  the  column 
of  mercury  shows  some  degree  of  contraction,  which  in  the  course 
of  a  year  amounts  to  as  much  as  a  degree  or  two.  Hence  it  is  best 
to  lay  newly  made  thermometers  aside  to  season  before  graduation 
in  order  to  provide  against  such  changes. 
After  the  requisite  time  has  elapsed,  the  graduation  is  accom- 
plished by  introducing  the  thermometer  into  a  vessel  of  finely 
broken  ice  and  allowing  it  to  remain  till  the  column  of  mercury  no 
longer  recedes,  it  is  then  pointed  off  by  making  a  nick  on  the  tube 
with  a  fine  file.  The  boiling  point  is  ascertained  by  placing  the 
thermometer  in  a  jacketed  vessel  which  is  filled  and  surrounded 
with  steam  supplied  by  boiling  water  in  the  bottom  of  the  inner 
section.  After  sufficient  time  has  elapsed  to  allow  the  whole  to 
become  fully  heated,  it  is  pointed  off  as  before  and  laid  aside  to  cool. 
The  space  between  these  two  points  is  then  divided  into  180  parts 
and  after  being  numbered  the  thermometer  is  ready  for  use.  As 
the  increase  in  the  volume  of  mercury  is  quite  uniform  in  propor- 
tion to  the  heat  added,  the  divisions  will  be  all  equal  when  a  perfect 
tube  is  used,  and  this  is  essential  in  making  a  standard  instrument. 
For  ordinary  thermometers  it  is  not  customary  to  calibrate  the  tubes, 
but  in  order  to  obtain  satisfactory  results  several  intermediate 
points  are  taken  with  the  aid  of  a  standard  and  the  required  divis- 
ions made  between  them.  Generally  the  points  selected  are  320, 
920,  1 5 2°  and  2120,  making  60  divisions  between  each  section,  and 
for  more  accurate  work  every  300  are  registered.  The  great  dis- 
crepancy found  in  the  thermometers  made  up  for  the  trade  is  from 
the  fact  that  the  intermediate  points  are  not  observed.  A  top  and 
a  bottom  degree  only  are  pointed  off  and  the  rest  of  the  scale  has 
to  take  chances  for  correctness.  So  when  it  is  considered  how  diffi- 
cult it  is  to  procure  even  a  small  amount  of  tubing  that  has  an  equal 
