196  COMMERCIAL  GLACIAL  PHOSPHORIC  ACID. 
To  determine  from  the  precipitate  the  quantity  of  anhydrous 
phosphoric  acid,  I  have  followed  two  ways  ;  if  its  quantity  was 
large,  I  have  dried  it  upon  a  brick  tile  at  ordinary  temperature, 
ascertained  its  precise  weight,  ignited  a  convenient  portion  of 
it,  and  then  calculated  the  weight  which  would  have  resulted 
from  the  ignition  of  the  whole.  If  the  quantity  of  the  precipi- 
tate was  small,  the  whole  of  it  was  ignited  together  with  the 
filter,  the  amount  of  ashes  of  which  was  ascertained  by  incinera- 
ting a  portion  of  the  same  sheet  of  paper,  and  deducting  the 
weight  of  ashes  from  the  whole  weight  obtained.  The  result  in 
either  case  is  pyrophospate  of  magnesia  2Mg  0,  P05,  which 
contains  63.33  per  cent,  of  P05. 
The  following  table  contains  in  the  first  column  the  amount  of 
phosphoric  acid  employed  ;  in  the  second,  the  weight  of  im- 
purities ;  third,  the  percentage  of  the  same  ;  fourth,  the  weight  of 
2MgO,  pP05;  fifth,  the  weight  of  P05 ;  sixth,  its  percentage 
of  the  pure  hydrate ;  and  in  the  seventhcolumn  the  percentage 
of  the  free  P05  in  the  specimen. 
I.  II.  III.  IV.  v.  VI.  VII. 
550  grs.  r45.0  grs.  8.1818  609.86  grs.  386.12  grs.  76-46  70.20 
100.75  «  0-8. "        -794    122.77  "      77.75      "  77.79  77.19 
48.20  »    none   c3.22         40.04  "  83.48  83.48 
Pure  metaphosphoric  acid  HO,  P05  contains  88.806  per  ct. 
P05,  and  the  best  specimen  which  I  have  examined  contains 
5.32  per  ct.  less,  while  between  the  highest  and  lowest  there  is  a 
difference  of  13.28  per  ct.  in  the  available  acid,  and  after  the 
deduction  of  the  earthy  phosphates  the  difference  is  still  7.02 
per  ct.  It  cannot  be  doubted  that  the  large  amount  of  impurities 
found  in  the  first  specimen  is  accidental,  and  probably  derived 
from  the  vessel  in  which  the  phosphoric  acid  was  fused ;  the 
purity  of  the  other  two  specimens  taken  out  of  the  same  jar, 
speak  in  favor  of  this  supposition,  and  it  appears  to  me,  on  ac- 
count of  this  great  difference,  as  if  most  likely  different  portions 
of  the  acid,  fused  at  the  same  time  in  one  vessel,  may  contain 
a  variable  proportion  of  impurities. 
From  the  amount  of  water  alone  no  conclusion  can  be  arrived 
at  as  to  the  proportion  of  the  three  phosphoric  acids  present  at  the 
same  time.  To  ascertain  this,  an  acid,  known  beforehand  not  to 
contain  any  impurities,  after  its  solution  in  cold  water  and  over- 
\ 
