( 252 ) 



Tlic same phenomenon presents itself as with silicic acid. The 

 more it is modifief], the more Avater it can absorb at the higher 

 vapour-tensions. The hysteresis is removed to that region. The 

 Gels II and III at rehydration can absorb up to more than 

 8 Mol. IlgO, while I can come no fnrthcr than to ± 3.6\ 



It is highly remarlvable that III requires months for it, II only 

 days. This too points to a difference in structure. Now it is very 

 I'emarkable that Gel III does not show any air-bubbles escaping 

 at tiie point Oq, when immersed in water, while Gel II docs. In 

 Gel II there appear immediately on the suiface and at the rims 

 of the transparent (so exceedingly thin) films at numerous places 

 smaller and larger air-bubbles. Consequently the gel contains cavi- 

 ties, hut their joint volume is not by far so great as in silicic acid. 

 In connection herewith seems to bo that II absorbs pretty quickly 

 a considerable amount of aqueous vapour at a vapour-tension of 12 

 m.m., while III, which contains no cavities, does this so extremely 

 slowly. 



It results from the above that no chemical hydrate can be obtai- 

 ned by drying the Gel by exposure to the air, and that moreover the 

 composition must change with the change in the vapour-tension of the 

 air. This was also proved by daily weighing a quantity of I exposed 

 to the air (freshly prepared and pressed between two porous plates) 

 for half a year. In the beginning it contained =t 18 Molec. HoO 

 on 1 Molec. Fe2 O3 and contained therefore much water inclosed. 

 With decreasing velocity the proportion had fallen after 9 days to 

 5,41120; from that period it came sensibly under the influence of the 

 changes in the vapour-tension of the air, so that it decreased irre- 



gularly 



weidit and sometimes increased. After VL month the 



