386 
CONVERSION  OF  THE  PHOSPHORIC  ACIDS. 
committee  having  charge  of  the  final  revision  of  the  Pharmaco- 
poeia, experienced  this  difficulty  and  handed  the  subject  over  to 
me  for  investigation. 
It  is  well  known  that  the  monobasic  acid  produces  gelatinous 
precipitates  in  the  solutions  of  most  metallic  oxides,  coagulates 
albumen,  and  after  neutralization,  yields  with  nitrate  of  silver  a 
white  precipitate  ;  the  deutohydrate  resembles  the  former  only 
by  precipitating  silver  salts  white  after  it  has  been  previously 
neutralized  ;  the  terhydrate  under  the  same  circumstance,  yields 
a  yellow  precipitate.  For  the  following  experiments,  Merck's 
glacial  phosphoric  acid  was  employed,  and  care  taken  to  select 
pieces  perfectly  transparent  and  free  from  earths  and  other 
acids. 
If  some  of  this  glacial  phosphoric  acid  is  thrown  into  cold  water, 
it  is  slowly  dissolved,  and  the  solution  shows  the  above  reactions 
of  the  monohydrated  acid  ;  set  aside  at  our  summer  temperature 
for  two  or  three  weeks,  the  solution  ceases  to  coagulate  albumen^ 
and  yields  now  a  purely  yellow  colored  precipitate  with  salts  of 
silver.  The  acid  has  been  converted  into  the  terhydrated,  and 
apparently  without  becoming  first  the  deutohydrate ;  for,  in  pro- 
portion to  the  decrease  of  the  coagulation  the  yellow  color  of 
the  precipitate  becomes  more  apparent.  The  change  is  not 
brought  about  suddenly,  but  gradually  ;  and  similar  is  the  be- 
haviour of  the  deutohydrate  ;  dissolved  in  water,  it  is  gradually 
converted  into  the  terhydrate.  It  is  very  probable  that  the  bulk 
of  the  solution  and  its  density,  that  is,  the  concentration  of  the 
solution,  may  have  a  marked  influence  on  the  time  requisite  for 
forming  the  terhydrate,  as  I  shall  show  in  another  place,  but  my 
experiments  were  not  extended  so  far. 
When  a  strong  solution  of  metaphosphoric  acid  in  water  is 
heated  to  the  boiling  point,  and  the  boiling  continued  with  the 
precaution  to  condense  the  evaporating  water  or  most  of  it  in 
the  retort,  so  that  the  solution  may  at  no  time  assume  a  syrupy 
consistence,  it  will  be  found  that  the  bulk  of  the  precipitate  , 
produced  in  a  solution  of  albumen  wnll  gradually  lessen,  and 
after  an  hour  or  two,  according  to  the  amount  operated  with, 
will  cease  entirely  ;  but  now  and  at  all  times  during  the  ebulli- 
tion, the  liquid,  neutralized  with  soda  or  ammonia,  produces  a 
white  precipitate  with  nitrate  of  silver,  free  from  any  tinge  of 
