of the Metallic Oxides. 97 



Ethylenic hydroxide, CH 3 . C(OH) 3 (or acetic acid, 

 C 2 H 4 2 -f H 2 0), when distilled, is dehydrated, and forms the 

 corresponding oxyhydroxide (i. e. the common acetic acid), 

 which is represented in the state of vapour, not by the formula 

 C 2 H 4 2 or CH 3 . CO . OH, but by (C 2 H 4 2 ) 2 or ' 



CH 3 .C(OH).O.C(OH).CH 3 . 



I — o-i 



Again, the transformation on prolonged distillation at the 

 ordinary pressure of certain polyatomic hydroxides, notably 

 glycerol, C 3 H 5 (0H) 3 , and its monochlorhydrines, C 3 H 5 (0H) 2 C1, 

 into condensation-products by dehydration are further examples 

 of this fact. 



Returning now to the dehydration of mineral hydroxides, 

 we have examples in the case of the acid hydroxides. 



1. Phosphoric Acid, H 3 P0 4 or (H0 3 )PO, when heated to 

 200°, is converted into pyrophosphoric acid, H 4 P 2 7 or 

 (HO) 2 PO . . PO(OH) 2 . Note also, in passing, the various 

 modifications of condensed metaphosphoric acid, discovered by 

 Fleitmann and Henneberg. 



2. Boric Acid, when dry and crystalline, is represented by 

 H 3 B0 3 . On heating, it dehydrates progressively as follows: — 



Dried at ordinary temperature . H 3 B0 3 . 



100° HB0 2 or H 2 B 2 4 . 



„ 160° H 2 B 4 7 . 



„ 270° H 2 B 16 25 . 



At higher temperatures there is total dehydration and for- 

 mation of fused anhydride (B 2 3 ) w . 



3. Silicic Acid. — Normal silicic acid, H 4 Si0 4 , is scarcely 

 known as such. It no doubt forms the gelatinous silica when 

 freshly precipitated. When heated, it exhibits progressive 

 dehydration: — 



Dried at ordinary temperature . H 2 Si0 3 or H 4 Si 2 6 . 



„ 20°-25 6 ..... H 2 Si 2 5 . 



„ 60° H 2 Si 3 7 . 



„ 80°-100° H 2 Si 4 9 . 



„ 250°-270° H 2 Si 8 17 . 



After which, total dehydration with formation of (Si0 2 ) n . 



4. Titanic Acid. 



Dried at ordinary temperature . H 4 Ti0 4 . 



„ overH 2 S0 4 H 2 Ti0 3 or H 4 Ti 2 6 . 



„ at 60° H 6 Ti 4 O n . 



„ at 100° H 2 Ti 2 4 . 



Also similarly with molybdic and stannic hydrates. 

 Phil. Mag. S. 5. Vol.* 20. No. 123. August 1885. H 





