Am.  Joub.  Phakm. 
Not.  1, 1871. 
502  Water  Air-pump  and  its  Uses, 
following  number  of  millimeters,  if  the  perpendicular  length  of  the 
discharge-pipe 
was — 
3  meters 
to 
130  millimeters. 
9  meters 
to 
526 
4  " 
200 
10  " 
580 
5  " 
280 
11  " 
6i 
642 
6  " 
320 
u 
12  " 
705 
7  " 
a 
400 
u 
13  " 
u 
715 
8  " 
u 
471 
a 
14  " 
u 
728 
With  the  above  diameter  and  greatest  length  of  the  discharge-pipe 
a  vacuum  corresponding  to  573  mm.  may  be  produced  by  10  litres 
water  in  a  vessel  of  the  same  capacity  within  10  minutes. 
The  above  results  will  be  obtained  only  by  regulating  several  times 
the  water  supply  cock,  so  that  the  glass  tube  inserted  in  the  discharge* 
pipe  will  show  air-bubbles  of  uniform  size  mixed  with  sufficient  quan- 
tities of  water. 
Evaporation,  Boiling  and  Distilling  in  vacuo, — The  boiling  point 
of  a  liquid  is  influenced  by  the  pressure  upon  it.  The  degrees  in  the 
scale  of  fig.  1  are  centesimal,  and  give  the  boiling  point  of  water  at 
a  pressure  indicated  by  the  figures  on  the  opposite  side,  the  barome- 
ter being  760  mm.,  =  28  Paris  inches.  With  the  pressure  of  the  air 
at  730  mm.,  =  27  inches,  water  will  boil  at  10,  15,  20,  30,  40,  60  and 
80°  C,  requiring  the  column  of  mercury  to  be  720-8,  717*3,  712*6, 
698*5,  675-1,  581*2  and  375*4  mm.  The  barometer  at  700  mm.,  = 
25*1  inches,  demands  for  the  same  purpose  a  mercury  column  of 
690*8,  687*3,  682*6,  668*5,  645*1,  551*2  and  345*4  mm. 
For  evaporation  in  vacuo,  the  authors  use  a  hemispherical  vessel, 
with  a  flat  rim,  and  of  sufficient  strength  to  withstand  the  great  pres- 
sure. They  use  as  a  cover  or  head  a  glass  vessel  of  similar  shape, 
uniform  in  thickness,  and  at  the  arch  lengthened  out,  so  that,  by  a 
heavy  rubber  pipe,  it  may  be  connected  directly  with  the  water-pump. 
The  contact  between  the  upper  and  lower  hemispheres  is  perfected  by 
a  rubber  gasket,  and  by  pressing  the  hand  upon  the  cover  for  a  min- 
ute or  two,  when  the  pump  begins  to  work.  The  vapors  of  the  evap- 
orating liquid  at  first  condense  upon  the  inside  of  the  glass  head,  and 
return  into  the  evaporating  basin ;  but  the  connecting-pipe  soon  be- 
comes sufficiently  warm  to  pass  the  vapors  without  condensation  into 
the  pump,  increasing  thereby  its  eflfects.  The  boiling  liquid  may  now 
be  easily  examined  through  the  glass  cover. 
