332 PERSALTS OF IRON. 



lost 9 grains of its weight. Hence, the consti- 

 tuents of this salt are 



1 L atom phosphoric acid 5-25 



1 atom peroxide of iron 5 



8 atoms water . . 9 



19-25 



6. Pertrisphosphate of iron. When the pre- 

 ceding salt is digested in caustic potash ley it 

 assumes a deep brownish red colour. The new 

 salt thus formed is tasteless and insoluble in 

 water. When heated to redness, the colour be- 

 comes darker, indeed almost black. I find, that 

 when 21 -875 grains of this salt, dried on the filter 

 in a gentle heat, are heated to redness in a pla- 

 tinum crucible, the loss of weight is 3*5 grains ; 

 which is equivalent to 3 atoms water very nearly. 

 I found that 50 grains of persesquiphosphate of 

 iron, when digested in potash ley, left 19 grains 

 of the red salt after exposure to redness ; 

 knowing the quantity of peroxide of iron in the 

 persesquiphosphate, it is easy to see that the red 

 salt is composed of 



1 atom phosphoric acid 3-5 



3 atoms peroxide of iron 15 



3 atoms water . 3-375 



21-875 



It is, therefore, a pertrisphosphate of iron. 



