ORGANIC  MATTER  OF  THE  AIR. 
445 
remaining  in  the  solution.  Perhaps  chlorate  of  potash  would 
have  been  better,  but  I  had  no  idea  at  the  time  of  the  difficulty 
afterwards  found  in  obtaining  the  same  quality.  A  solution  of 
this  manganate  was  made  in  pure  water,  and  carbonic  acid 
passed  through  until  a  reddish  purple  shade  was  obtained.  It 
was  then  tested  by  oxalic  acid,  adding  three  or  four  drops  of 
pure  sulphuric  acid.  The  purest  water  obtainable  was  added  to 
dilute  it  to  the  proper  amount.  This  often  failed;  and  I  have 
sometimes  for  a  whole  week  failed  to  obtain  the  proper  solution. 
Although  I  call  it  permanganate,  it  is  not  entirely  so  ;  it  is  a 
mixture  of  manganate  and  permanganate.  A  permanganate  of 
the  strength  described  has  a  dingy  appearance  and  uncertain 
color.  I  do  not  doubt  that  a  pure  permanganate  of  a  suitable 
strength  may  be  obtained  pleasant  to  work  with.  There  is  some 
difficulty  in  obtaining  pure  water  for  preparing  the  solution.  If 
allowed  to  stand  for  some  time  with  a  manganate  it  becomes 
purified. 
The  solution  of  chameleon  is  apt  to  change,  although  slowly, 
even  when  it  is  hermetically  sealed  in  a  glass  tube.  The  solu- 
tion described  had  become  nearly  colorless  when  sealed  up  her- 
metically for  about  three  months.  It  is  found  readily  to  change 
when  it  is  exposed  to  air  by  frequent  removal  of  the  stopper  of 
the  bottle  containing  it.  Its  strength  must  be  tested  occasion- 
ally ;  and  if  it  differs  from  the  standard,  a  calculation  must  be 
made  for  its  reduction.  The  strength  of  the  permanganate  solu- 
tion is  extremely  small.  A  few  grains  of  the  ordinary  solutions 
of  manganese  used  will  make  some  thousand  grains  of  the  solu- 
tion here  employed.  The  reason  of  this  lies  in  the  extremely 
small  amounts  of  organic  matter  found  in  even  the  worst  air. 
The  vessel  used  is  simply  a  bottle  with  a  perforated  stopper, 
through  which  pass  two  tubes.  To  one  of  these  a  stopcock  is 
attached,  to  the  other  a  clasp  or  stopcock.  The  standard  size 
proposed  is  100  cubic  inches ;  and  to  this  all  the  experiments 
have  been  reduced  ;  the  vessels  actually  used  contain  between  80 
and  100  cubic  inches  of  air.  The  stopcock  is  of  glass,  or  of  hard 
caoutchouc,  which  is  still  better.  When  the  bottle  is  to  be  filled 
with  the  air  to  be  tested,  the  stopper  is  removed,  and  the  pipe 
of  an  exhausting  pump  is  inserted,  reaching  to  the  bottom  of  the 
bottle.    The  pump  is  made  like  a  cylindrical  bellows  of  about  8 
