Am.  Jour.  Pharm.  \ 
February,  1904.  J 
Thermometers. 
53 
freezing  point  can  be  determined  with  great  accuracy,  for  water 
containing  ice  will  remain  at  the  same  temperature  just  as  long  as 
there  remains  any  ice  to  be  used  up,  any  accession  of  heat  being 
used  in  melting  the  ice.  The  boiling  point  of  water,  however,  is  not 
so  reliable,  but  being  somewhat  changeable  its  determination  is 
attended  with  greater  difficulty.  The  density  of  the  atmosphere  or 
barometric  pressure  has  considerable  effect  upon  it ;  the  purity  of 
the  water  and  the  nature  of  the  vessel  used  also  produce  more  or 
less  deviation,  and  have  to  be  taken  into  account  by  the  maker  who 
desires  to  furnish  reliable  instruments.  However,  these  difficulties 
can  all  be  overcome,  and  with  proper  care  and  allowances  these 
variations  can  be  corrected.  In  making  a  thermometer  it  is  of  great 
importance  to  select  a  piece  of  tubing  that  has  been  evenly  drawn 
throughout  its  entire  length.  If  at  any  part  the  bore  should  be 
enlarged  the  column  of  mercury  will  become  shortened  and  will 
register  too  low,  likewise  if  the  bore  should  be  contracted  the  col- 
umn will  be  lengthened  and  will  register  too  high;  hence  the  great 
necessity  of  being  sure  that  the  tube  has  an  equal  bore  at  every 
point.  This  important  fact  is  determined  by  introducing  a  small 
amount  of  mercury,  just  enough  to  make  a  short  length,  say  about 
2  inches,  and  then  moving  it  slowly  from  one  end  of  the  tube  to  the 
other  and  measuring  it  as  it  travels  along.  If  it  remains  the  same 
length  throughout,  the  tube  is  fit  for  use  and  is  sealed  over  at  the 
ends  for  protection  against  dust  and  moisture. 
Now  it  must  not  be  supposed  that  the  maker  of  thermometers 
goes  to  all  this  trouble  and  time  with  all  the  instruments  he  turns 
out.  Nor  is  it  necessary  in  all  cases,  but  only  when  making  a 
standard  by  which  others  are  compared,  as  will  be  hereafter  ex- 
plained. Having  selected  the  proper  tubing,  a  bulb  ot  the  required 
size  to  hold  the  mercury  is  blown  on  one  end.  This  operation 
requires  considerable  skill  and  practice  on  the  part  of  the  glass 
blower,  for  a  bulb  that  is  too  large  will  make  the, scale  too  extended, 
and  if  too  small  will  cause  the  degrees  to  be  too  close  together. 
A  bulb  also  is  blown  on  the  other  end  of  the  tube  for  the  purpose 
of  filling.  By  means  of  a  small  funnel  a  sufficient  amount  of  mer- 
cury is  introduced  into  the  upper  bulb.  Heat  is  then  applied  to  the 
lower  bulb  to  drive  out  a  portion  of  the  air  and  this,  when  allowed 
to  cool,  becomes  partially  filled  with  the  mercury.  Heat  is  again 
applied  to  the  bulb  till  the  mercury  boils  and  is  driven  out,  carrying 
