( 249 ) 



Chemistry. — „ On II i/drof/d of oxide of iron (ferric-oxide )''\ 

 By Prof. J. M. van Bkm.mklen. 



In the Rocueil dcs Travaiix Chimi(]iics des Pays-bns ^) Mr. Spring 

 stated some time ago that he had prepared a liydrate of oxyd of 

 iron : d'une eomposition définic. He had got it by allowing to dry — 

 by exposure to the air — the gelatinous precipitate produced by 

 ammonia in a diluted ferri-solution and that after having washed 

 it out. After 72 days the equilibrium became stable, and after 5 

 months it was found to be unchanged. It answered to the composi- 

 tion FcoO;;. 4TToO. Above sulphuric acid it lost water so that after 

 3 days the composition was Fcj O3 1.78 Hj (calculated by me). 



Formerly, in 1S88~) and 1892, I have communicated, that the 

 gelatinous oxide of iron had do stable composition, but was an 

 absorption-alloy of Fcg 0... with water. I had also found the number 

 ± 4,0 H2 in one of my preparations, but I considered it as an 

 alloy, that only answered to a whole number by chance. The opinion 

 that a fixed hydrate had been obtained, is contrary to the nature 

 of hydrogels. For the composition is continuously dependent on: 1". 

 the structure of the colloid, which in its turn depends upon tlie 

 circumstances of the gel-formation; 2'"'. the modifications that th(! 

 structure has undergone by the further ireatment; 3"'. the concentration 

 of the gaseousphase, with which the gel co-exists; 4"'. the temperature. 



Owing to my having determined the composition only with a few 

 vapour-tensions, Spring's communication led me to determine the entire 

 isotherm at 15° viz. of dehydration of rehydration and of rede- 

 hydration and to examine carefully how the Hydrogel behaves when 

 exposed to the air, the vapour-tension of which is continually changing. 



Accordingly I prepared again a Hydrogel from a higldy diluted 

 ferric-solution •') and worked with it at once, while it was still 

 (juite fresh. Besides I determined the same curves for the prepara- 

 tion already investigated by me in 1882, which had now grown 

 IG years older (III); and in the third place for a preparation made 

 in 1891, which had been under water for seven years (II). These 

 determinations were made in the same n anner as formerly for the 

 hydrogel of Si 0^ and of Cu O ';. On the graphic sketch I have 



Ï) 18.222 (1898). 



-) liecueil VK 100— U-i. J. f. pvakt. Ch. 40-529. 



') 1 Part Fej O^, on 66 p. water — 1 Mol. IVj 0^ on 585 p. water. 



■*) Vide Verslag Kon. Akad. of 28 Janr. 189.3 mul 29 .lune 18Ü5. At present tlio eqiiili- 

 brinuis are determined at vapour-pressures of 12" — II" — 10" — 10 — 9' — 81— 7'- — (J- — 5 

 — 4' — 2 — 0'^ ui.ra. both for A and for Z ^ and Z j". 



