IRON. 345 



num crucible to a heat of between 500 and 600. 

 The loss of weight was 7*6 grains. 



Now, as 1 7*375 grains of this salt contain 

 just 5 grains of sulphuric acid, which is equiva- 

 lent to the weight of an atom, the water of 

 crystallization ought likewise to be a determinate 

 number of atoms. Now, 7*875 grains would be 

 equivalent to 7 atoms. And the water actually 

 found was 7*6 grains, which is only 0-275 grains, 

 or ^th of an atom less than 7 atoms. 



But all the water cannot be separated from 

 this salt by heat, without at the same time de- 

 composing and separating a portion of the sul- 

 phuric acid. Sulphurous acid begins to be 

 disengaged, and sulphuric acid in a liquid state 

 (and therefore containing water) trickles down 

 the sides of the retort. 



It is easy to show that the water which re- 

 mains behind, and which comes over united to 

 the sulphuric acid, is exactly ith of an atom. 

 For the sake of explanation, I shall take for 

 granted what will be proved in the subsequent 

 part of this section ; namely, that an atom of 

 iron weighs 3 '5, an atom of protoxide of iron 

 4'5, and an atom of peroxide of iron 5. 



When protosulphate of iron, freed from as 

 much water of crystallization as possible, is 

 strongly heated, the protoxide of iron which it 

 contains is converted into peroxide at the ex- 

 pense of the sulphuric acid, which gives off one 



