388  CONVERSICN  OF  THE  PHOSPHORIC  ACIDS. 
trated  one,  and  apparently  without  the  previous  production  of 
the  bibasic  acid. 
At  first,  I  supposed  that  the  metaphosphoric  acid  contained 
some  compound  requiring  oxidation,  before  the  change  could  be 
effected,  but  a  careful  trial  showed  the  absence  of  any  nitrous 
acid  vapors  by  the  time  the  conversion  was  complete.  It  is 
likewise  not  the  presence  of  another  mineral  acid  which  effects  the 
change  of  the  mono-  into  the  terhydrate  ;  for  when  boiled  with  hy- 
drochloric acid,  the  solutions  scarcely  yield  more  of  the  terhy- 
drate than  is  obtained  by  the  aqueous  solution  alone,  while 
pyrophosphoric  acid  remains  mixed  with  it,  and  an  addition  of 
common  phosphoric  acid  does  not  exert  any  influence  on  the 
solution  provided  it  be  perfectly  free  from  nitric  acid. 
That  the  cause  cannot  be  looked  for  in  the  higher  temperature, 
necessarily  produced  by  the  addition  of  nitric  acid,  is  evidenced 
by  the  facts  that  the  more  concentrated  solutions  of  the  glacial 
phosphoric  acid  yield  no  terhydrate  by  boiling,  while  the  diluted 
solutions  boiling  at  a  lower  temperature,  yield  some,  and  that 
after  the  addition  of  nitric  acid,  the  liquid  need  be  but  heated 
to  near  its  boiling  point  to  effect  the  change  completely  though 
more  slowly  than  by  boiling.  The  density  of  the  solution,  as 
will  be  seen  from  the  above  statements,  exerts  a  strong  influence 
in  this  process  ;  but  nitric  acid  converts  both  concentrated  and 
diluted  solutions  into  the  common  phosphoric  acid. 
It  will  require  more  and  very  careful  experiments  to  determine 
precisely  the  way  in  which  nitric  acid  acts  in  this  case.  That 
nitric  acid  merely  acts  as  a  catalytic  agent,  seems  probable,  from 
the  absence  of  nitrous  acid  vapors  ;  but  it  might  be  possible  that  a 
compound  between  metaphosphoric  and  nitric  acid  is  formed  for  a 
short  time,  and  decomposed  again  into  nitric  acid  and  the  common 
phosphoric  acid  ;  or  the  monohydrated  acid  might  be  oxidized  to 
a  still  unknown  oxide,  POg  or  PO^, which  is  instantly  decomposed 
into  common  phosphoric  acid  and  oxygen,  the  latter  in  its 
nascent  state  uniting  with  NO^  or  NO3  to  form  nitric  acid  again. 
Certain  it  is,  that  a  small  quantity  of  nitric  acid  will,  with  pro- 
per precaution  to  prevent  its  evaporation,  change  a  considerable 
amount  of  metaphosphoric  acid ;  and  it  acts  quite  or  nearly 
as  quickly  upon  the  pyrophosphoric  acid. 
Graham,  though  he  does  not  state  so,  undoubtedly  operated 
