20         CHEMICAL  AND  PHARMACEUTICAL  MANIPULATIONS. 
a  hinge,  it  can  be  easily  put  in  proper  position  around  the  matters  to  be 
pressed  (after  the  latter  has  been  placed  in  the  box),  and  fastened  by  means 
of  the  pin.  The  ribs  on  the  outer  circumference  of  this  band  project  against 
the  inner  sides  of  the  box,  and  form  intermediate  grooves,  through  which 
the  expressed  liquid,  issuing  from  the  holes,  can  readily  pass  off  into  the 
receiving  vessel  at  the  spout  beneath. 
To  put  the  press  in  action,  the  lever  being  upheld  by  the  pin  at  h,  the 
winch-handle  is  turned  to  the  left,  in  order  to  lower  the  rack  and  follower, 
until  the  latter  presses  upon  the  wooden  block  b,  Fig.  376,  which  caps  the 
material  under  pressure.  The  lever  is  then  raised,  and  the  pall  allowed  to 
work  into  the  ratchet,  which  will  cause  the  latter  to  turn,  and  produce  the 
descent  of  the  rack.  This  is  repeated,  if  requisite,  until  a  considerable 
pressure  is  obtained  ;  and  should  it  be  desired  to  go  on,  the  lever  is  elevated 
considerably  above  the  horizontal  line,  and  left  to  follow  the  consolidation 
of  the  contents  of  the  bag.  If,  however,  this  is  unnecessary,  the  pin  h  is 
inserted,  upon  which  the  lever  remains.  The  amount  of  pressure  is  also 
regulated  by  the  disposal  of  the  weight  m  in  the  various  notches  of  the 
lever. 
When  it  is  not  expedient,  as  in  the  case  of  pulpy  and  similar  matters,  to 
press  the  substance  in  the  box  without  first  enveloping  it  in  a  cloth,  it  may 
be  wrapped  in  unbleached  Russia  canvass  ;  and  the  bag-shape  bundles  thus 
formed  placed  in  the  box,  with  a  stiff  plate  of  tinned  copper  interposing 
every  two  of  them.  They  should  be  folded  so  as  not  to  make  a  thickness  of 
more  than  an  inch. 
As  the  cloths  absorb  a  considerable  quantity  of  the  expressed  juice,  and 
occasion  loss,  the  pressing  should  be  accomplished  without  them  in  all 
possible  cases.  The  cloths,  used  for  confining  substances  from  which  oily 
liquid  is  to  be  expressed,  must  be  woollen  and  thick. 
>;<  *  *  #  >fc 
Kemp's  I^iermostat. — When  street  gas  is  used  for  heating  the  air-baths, 
it  is  apt  to  give  unequal  temperatures,  owing  to  the  variable  pressure  upon 
the  service-pipes  at  different  times.  To  prevent  this  annoyance,  Kemp  has 
devised  a  most  convenient  and  efficient  apparatus,  which  he  properly  desig- 
nates a  Thermostat,  as  it  regulates  the  supply  of  the  gas  to  the  burner,  and 
of  course  the  amount  of  heat  thus  applied  to  the  substance  under  process, 
thereby  insuring  a  constant  temperature  for  any  length  of  time. 
This  simple  and  ingenious  apparatus  will  be  found  serviceable  for  all 
operations  requiring  a  prolonged  temperature  of  great  uniformity.  The 
author  used  it  successfully  in  promoting  tedious  fermentations,  artificial 
incubation,  and  for  obtaining  products  of  the  decomposition  of  organic 
bodies  at  fixed  temperatures.  The  use  of  mercury  renders  it  available  only 
for  temperatures  below  the  boiling-point  of  that  metal;  but  by  making  the 
instrument  of  iron  and  substituting  fusible  alloys  for  mercury,  it  becomes 
applicable  for  higher  degrees. 
