512 
Monocalcium  Phospliate. 
Am.  Jour.  Pharm. 
Oct.,  1890. 
The  same  explanation  holds  in  all  cases  where  the  aluminium 
does  not  readily  enter  into  a  reaction  which  would  be  exothermic. 
The  surface  of  the  metal  becomes  covered  with  a  thin  layer  of  gas, 
which  prevents  further  change,  but  which  can  be  removed  by  reduc- 
tion of  pressure  or  by  the  roughening  of  the  surface  of  the  alumin- 
ium in  consequence  of  the  deposition  of  another  metal  upon  it. 
The  action  of  nitric  acid  and  nitrates  on  aluminium,  like  the 
action  of  sulphuric  acid  and  sulphates,  is  impeded  by  the 
formation  of  a  film  of  gas  on  the  surface  of  the  metal.  If  the 
action  takes  place  in  a  vacuum,  or  if  the  liquids  are  heated, 
dissolution  of  the  metal  takes  place  much  more  rapidly.  Very 
dilute  nitric  acid  yields  nitrogen,  nitric  oxide  and  ammonia.  Nitirc 
acid  of  3  per  cent.,  mixed  with  a  small  quantity  of  platinum  chlo- 
ride, dissolves  aluminium  with  very  slight  evolution  of  gas  and 
formation  of  considerable  quantities  of  ammonium  nitrate.  Alu- 
minium dissolves  in  a  solution  of  the  normal  nitrate,  especially  on 
heating,  with  evolution  of  hydrogen  and  formation  of  the  basic 
nitrate  2Al203,N205,ioH20.  At  the  ordinary  temperature,  dissolu- 
tion takes  place  very  slowly,  but  the  action  is  somewhat  more  rapid 
in  a  vacuum.  The  decomposition  of  water  by  aluminium  would 
result  in  the  formation  of  the  hydroxide  and  hydrogen,  either  of 
which  can  form  a  protecting  film  on  the  surface  of  the  metal.  In 
presence  of  any  salt  which  will  dissolve  the  aluminium  hydroxide, 
the  metal  is  gradually  attacked.  Salts  which  do  not  dissolve 
alumina  have  no  influence  on  the  change. 
MONOCALCIUM  PHOSPHATE.1 
By  I.  Stoklasa. 
Monocalcium  phosphate,  CaH4(POJ2  -f-  H20,  was  prepared  by 
dissolving  pure  dicalcium  phosphate  in  31  per  cent,  phosphoric 
acid,  and  recrystallizing.  It  was  then  washed  with  absolute  alco- 
hol and  ether,  and  dried.  Washing  with  ether  only  (Erlenmeyer, 
Birnbaum,  Wattenberg)  never  removes  the  free  phosphoric  acid 
completely,  so  that  if  prepared  in  this  way  it  may  contain  several 
per  cent,  of  free  phosphoric  acid.  The  presence  of  the  latter  may  be 
proved,  either  by  treatment  with  absolute  alcohol,  or  by  dissolving 
the  salt  completely  in  water  and  estimating  volumetrically  with  deci- 
1  Listy  Chem.,  13,  pp.  203,  240,  273.    Reprinted  from  Jour.  Chem.  Soc,  July, 
1890,  p.  695. 
