Am'octu!ri8^6arm*}       Determination  of  Melting  Points.  487 
sider  the  different  methods  are  as  follows :  Class  I,  Camphors,  resins, 
waxes  and  alkaloids ;  Class  II.  Fats ;  Class  III.   Substances  not 
readily  oxidized ;  Class  IV.  Substances  easily  oxidized  in  contact  with 
-  air. 
Class  I.  Camphors,  resins,  waxes  and  alkaloids.  A  convenient 
and  practical  method,  which  I  would  advise  being  used  for  this  class 
is  the  following :  A  small  beaker  is  filled  two-thirds  full  of  mercury, 
into  which  a  thermometer  is  suspended  so  that  the  mercury  just 
covers  the  bulb  of  the  thermometer.  A  small  quantity  of  the  article 
under  examination  is  placed  directly  on  the  mercury  and  kept  as  close 
to  the  thermometer  as  possible.  Heat  is  then  gradually  applied  so 
that  the  rise  of  the  mercury  in  the  thermometer  does  not  exceed  two 
or  three  degrees,  Fahrenheit,  a  minute.  When  the  first  indications  of 
melting  are  visible,  note  the  number  of  degrees  and  carefully  regulate 
the  heat  so  as  to  keep  the  temperature  stationary  at  this  point  for  a 
little  time  to  see  if  the  whole  quantity  will  melt.  If  the  article 
under  examination  does  not  melt  completely,  again  apply  heat  very 
carefully  until  the  whole  quantity  is  melted  to  a  transparent  liquid. 
The  temperature  at  which  this  takes  place  may  be  regarded  as  the 
melting  or  fusing  point  near  enough  for  all  practical  purposes,  though 
if  greater  accuracy  is  required  it  is  first  necessary  to  examine  a  body 
with  a  known  melting  point  under  the  same  conditions  as  are  those 
when  examining  the  body  with  the  unknown  melting  point.  We 
obtain  thus  the  ratio  of  variation  in  this  particular  instance,  and  the 
required  melting  point  can  then  easily  be  obtained  from  the  given  data. 
This  comparison  can  be  used  in  all  subsequently  named  classes. 
Class  II.  Fats.  The  above  method  can  be  used  here  also,  but  it  will 
not  give  such  satisfactory  results  as  the  following:  A  small,  thin- 
walled  glass  tube  is  drawn  out  very  fine,  and  the  melted  fat  then 
sucked  up  into  the  tube,  the  capillary  portion  of  which  is  then  mel- 
ted to  a  point.  The  part  to  be  used  is  then  laid  aside  for  two  or 
three  days  in  a  cool  place.  The  tube  is  then  attached  to  a  thermome- 
ter by  means  of  a  rubber  band,  in  such  a  manner  that  the  capillary 
portion  is  as  close  to  the  bulb  of  the  thermometer  as  possible.  The 
apparatus  is  now  placed  so  that  the  liquid,  either  water  or  sulphuric 
acid,  just  covers  the  bulb  of  the  thermometer.  Now  gradually  apply 
heat,  being  careful  about  the  rise  of  the  mercury  in  the  thermometer. 
The  melting  point  is  that  degree  at  which  the  fat  becomes  transparent. 
Another  method  is  to  have  the  capillary  tube  open  below,  and  noting 
