272  Methods  of  Studying  Coal.        {  Aj1u^eUrI'9?7harm' 
sharp  and  heavy  knife  is  employed  for  sectioning  and  its  edge  must 
be  kept  moistened  with  ordinary  strong  alcohol.  The  sections  are 
turned  back  on  the  knife,  as  they  are  sliced,  by  means  of  a  large 
camel's  hair  brush,  wet  with  alcohol.  Successful  sections  must  usu- 
ally be  five  micromillimeters  or  thinner.  If  the  processes  have  been 
successfully  carried  out,  abundant  and  consecutive  slices  can  easily 
be  secured,  showing  every  feature  of  organization  of  the  coal. 
After  the  sections  are  cut  they  are  dehydrated  by  means  of  abso- 
lute alcohol,  to  which  a  quantity  of  chloroform  has  been  added  to 
obviate  the  softening  of  the  nitrocellulose  in  the  coal.  From  the 
absolute  alcohol  and  chloroform  they  are  transferred  to  benzole  or 
some  other  clearing  medium  and  are  then  mounted  in  hard  Canada 
balsam,  dissolved  in  benzole  or  whatever  clearing  agent  has  been 
used  on  the  sections.  Where  too  high  a  degree  of  clearing  is  unde- 
sirable, as  for  example  in  the  case  of  oil  shales,  chloroform  may 
with  advantage  replace  benzole  or  xylol.  After  the  covers  are  put 
on,  the  preparations  are  allowed  to  dry  for  a  day  in  a  horizontal 
position  and  they  are  gradually  warmed  up  with  lead  weights  on  the 
covers  to  promote  flattening.  When  the  balsam  has  become  so  thick- 
ened by  the  heat  as  to  set  in  the  cold,  the  slides  are  cleaned  up. 
Where  it  is  necessary  to  make  photomicrograms  of  them,  they  are 
still  further  flattened  by  means  of  a  clip  clothes  pin  acting  on  a  disk 
of  cork  (over  the  cover)  in  the  heat  of  a  warm  bath.  For  pho- 
tographic reproduction,  the  best  lenses  (Zeiss  apochromatics)  are 
desirable  and  these  should  be  used  with  a  yellow  screen  and  chromatic 
plates.  Screens  of  other  colors,  although  theoretically  more  desirable 
than  yellow,  have  not  been  found  practically  to  give  as  good  results, 
probably  on  account  of  the  difference  between  the  visual  and  chem- 
ical focus  even  in  the  best  microscopic  lenses.  The  largest  possible 
amount  of  light  should  be  used,  an  end  to  be  attained  both  by  having 
a  powerful  electric  arc  as  a  source  of  illumination  and  the  diaphragm 
of  the  sub-stage  condensor  opened  to  the  widest  possible  degree,  con- 
sistent with  sharp  focusing  of  the  object.  Naturally  only  the  very 
best  lenses  will  give  good  results  under  these  conditions.  The 
details  of  photomicrography  are  so  familiar  to  all  scientific  workers 
in  the  field  here  described,  that  further  details  will  only  add  unduly 
to  the  length  of  this  article. 
In  conclusion  are  added,  at  the  editor's  request,  some  statements 
in  regard  to  the  bearing  of  the  results  obtained  by  the  technical 
manipulations  described  upon  the  problem  of  the  mode  of  formation 
of  coal.    It  is  to  be  noted  that  the  mass  of  expert  opinion  at  the 
