of the Metallic Oxides, 99 



5. Ferric Hydrates. — The following hydrates of ferric 

 oxide, Fe 2 3 , are known: — 



Normal hydrate, 



H 6 (Fe 2 )0 6 . 



. Limonite, 



Fe 2 3 



3H 2 0. 



n ii 



H 4 (Fe 2 )0 5 . 



ii m 



Fe 2 3 



. 2H 2 0. 



ii ' ii 



H 2 (Fe 2 )0 4 . 



. Gothite, 



Fe 2 3 



H 2 0. 



*i ii 



H 6 (Fe 2 ) 2 9 . 



. Limonite, 



2Fe 2 3 



3H 2 0. 



ii ii 



H 2 (Fe 2 ) 2 7 . 



. Turgite, 



2Fe 2 3 . 



H 2 0. 



ii ii 



H 10 (Fe 2 ) 3 O 14 . 



ii 



3Fe 2 3 



5H 2 0. 



On heating, they are all dehydrated to Fe 2 3 . 



The brown ochreous precipitate obtained on adding an 

 alkali to ferric chloride is undoubtedly Fe 2 (OH) 6 , but when 

 dried in a vacuum it has the composition (Fe 2 ) 2 H 6 9 , or 

 2Fe 2 3 . 3H 2 0. Heated to ebullition, it dehydrates and 

 becomes Fe 2 H 2 4 or Fe 2 3 , H 2 0. By prolonged ebullition 

 it can be totally dehydrated. 



6. Aluminium Hydrates. — Aluminium solutions, on addition 

 of ammonia in excess, give the normal hydrate Al 2 (OH) 6 in 

 the form of a white gelatinous precipitate. On prolonged 

 ebullition this is partially dehydrated, like the corresponding 

 ferric hydrate, and becomes insoluble in acids and in alkalies, 

 like calcined alumina (Pean de St. Grilles). This new hy- 

 droxide corresponds to the formula (H0) 2 A1 2 . . Al 2 (OH) 2 , 

 which is analogous to pyrophosphoric acid. Besides the 

 normal hydrate Al 2 (OH) 6 , hydrargillite, there is also the 

 mineral diaspore, Al 2 2 (OH) 2 or A1 2 3 . H 2 0, to which we 

 shall refer hereafter. 



All these oxyhydroxides, both natural and artificial, which 

 are products of the condensation of the normal hydrates, are 

 completely dehydrated under the action of heat, at a tempe- 

 rature above that at which they are formed. 



Since the same causes produce the same effects, it is natural 

 to conclude that the last phase of the dehydration is accom- 

 panied, as in those which precede it, by an analogous molecular 

 condensation. The elimination of water having determined 

 the accumulation of the radicals in the products to which 

 they give rise, it would be unreasonable and illogical to 

 suppose that an entirely different phenomenon occurred at 

 the moment of elimination of the last molecule of water, 

 resulting in the dislocation of these residues of condensation 

 previously soldered together by the intermediatory oxygen. 

 \ Delrydration and condensation are intimately connected. 

 We must conclude, therefore, that at the moment the last 

 molecule of water is given off, not only does the previous 

 I condensation of the oxyhydroxide remain, but that, con- 

 ' formably to the general rule, it becomes still more accentuated, 



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