N. M. Comber 379 



showed an increment in the amount of iron dissolved in dilute acid after 

 partial ignition. 



(a) Ferric oxide or hydroxide was obtained in a variety of ways. 

 Two purchased samples (one red and one brown), the dried precipitates 

 obtained by adding NH4OH, NaOH, and KOH to solutions of FeClj both 

 in hot solution and in the cold, and the gels obtained by evaporating 

 ferric hydroxide sols were all used. They showed a continuous decrease 

 in the solubility of iron in N/5 HCl during ignition. 



(6) Three 50 gm. portions of ferric oxide were weighed out. One was 

 shaken with 100 c.c. di.stilled water and then treated with 200 c.c. 

 Ca(0H)2 solution. Another was shaken with 100 c.c. silica sol (containing 

 2-085 gm. SiOa in the 100 c.c.) and precipitated with 200 c.c. Ca(0H)2 

 solution. The remaining portion was shaken with 100 c.c. silica sol and 

 200 c.c. distilled water were added. Each of these systems was then 

 evaporated to dryness on the water bath, and the relative amounts of 

 iron extracted by N/5 HCl before and after 2 minutes' ignition was 

 observed. The ignition decreased the amount of iron dissolved where 

 only Ca(0H)2 had been added, but increased it about threefold in the 

 two other cases. 



The amount of iron dissolved from the unignited material in which 

 silica had been used was very much less than where only Ca(0H)2 was 

 used. 



When the silica sol or the precipitate formed by adding Ca(0H)2 

 thereto, was evaporated separately from the ferric oxide and then in- 

 timately mixed with it, there was no increase by ignition, but always a 

 decrease, in the amount of iron dissolved by dilute acid. 



These experiments were repeated many times with three different 

 samples of ferric oxide, and with similar results. 



An increase in the amount of iron dissolved by dilute acid was also 

 observed in the products obtained by suspending ferric oxide in a dilute 

 solution of waterglass and adding a solution of AICI3 , and by suspending 

 ferric oxide in AICI3 and adding NH4OH. 



Wherever the ferric oxide particles were cemented together by 

 gelatinous precipitates, a partial ignition increased the amounts of iron 

 dissolved by acid, just as is observed in soils. 



