504  Water  Air-pump  and  its  Uses.  {^No^vTim^''" 
through  the  cocks  connected  with  the  recipient,  thereby  decreasing 
the  partial  vacuum. 
The  vapors  of  the  evaporating  liquid  may  be  condensed  by  con- 
necting the  basin  with  the  pump,  instead  of  from  the  dome  of  the 
cover,  from  the  pipe  of  the  recipient,  and  inserting  a  suitable  flat- 
lipped  vessel,  as  arranged  in  fig.  2,  the  stop-cocks  in  the  pipe  being 
then  unnecessary.  The  air  is  now  exhausted, through  this  cooler, 
which  is  immersed  in  cold  water  and  receives  the  condensed  liquid. 
Should  the  liquid  to  be  evaporated  be  of  a  larger  volume  than  the 
capacity  of  the  basin,  the  latter  is  filled  from  time  to  time  through 
a  pipe  connected  with  the  cover,  by  opening  the  stop-cock,  when  the 
atmospheric  pressure  will  force  the  liquid  from  the  supply  vessel  into 
the  basin  without  disturbing  the  operation.  Large  quantities  may 
thus  be  evaporated  from  relatively  small  basins  without  destroying 
the  vacuum. 
The  authors  regard  flat  evaporating  basins,  not  materially  departing 
from  the  spherical  or  elliptic  shape,  as  the  most  suitable  forms,  and 
caution  against  the  employment  of  flat-bottomed  vessels.  The 
strength  of  these  vessels  must  be  sufficiently  great  to  withstand  the 
pressure.  A  perfectly  exhausted  vessel,  the  barometer  being  760  mm., 
=  28  Paris  inches,  sustains  a  pressure  of  2*066  lbs.  upon  each  square 
centimeter  surface.  A  basin  of  5  litres  capacity  has  a  surface  of 
960,  the  recipient  910,  and  the  glass  head  of  980,  the  whole  appara- 
tus 2850  cm.  surface,  which  would  have  to  sustain  a  pressure  of 
5880  lbs.  Since,  however,  evaporation  is  in  this  apparatus  really 
performed  at  a  pressure  of  730  mm.,  =  27  inches,  the  weight  to  be 
sustained  is  reduced  to  5670  lbs.  The  authors  have  used  tinned  cop- 
per and  block  tin  basins  with  success,  also  glass  basins  of  3  litres 
capacity,  and  they  are  experimenting  with  porcelain.* 
Filtration  in  Vacuo. — It  is  exceedingly  difficult  to  regulate  the 
working  of  the  water  air-pump  so  as  not  to  produce  too  excessive  a 
pressure  for  the  filter  to  sustain.  A  stop-cock,  that  may  be  intro- 
du-ced  between  the  vessel  and  the  pump,  would  but  partly  fulfill  the 
desired  object,  because  it  does  not  allow  to  obtain  any  desired  degree 
of  partial  vacuum.  The  authors  constructed  a  simple  regulator,  con- 
sisting of  a  metallic  pipe,  inserted  at  the  place  mentioned,  to  which  a 
curved  handle  is  soldered,  supplied  with  a  screw  ending  into  a  fine 
*  Porcelain-lined  iron  basins  might  answer  for  many  purposes. — J.  M.  M. 
