98 Prof. Louis Henry on the Polymerization 



It is well to state here that I do not attach an absolute 

 value to the formulae attributed to a good number of these 

 incomplete hydrates of mineral acids. It would be difficult 

 to regard all of them as definite compounds. The fact which 

 is in evidence, and with which alone we are at present con- 

 cerned, is that with rise of temperature the dehydration 

 goes on, and the molecular composition becomes more and 

 more complicated by the accumulation of radicals in the oxy- 

 hydroxyl residue. Though these hydrates themselves may 

 not always have the character of well-defined compounds, 

 they are often represented by corresponding derivatives 

 having all the characteristics of true chemical substances. 



It is sufficient to mention Troost and Hautefeuille's oxy- 

 chlorides of silicon, the numerous polysilicates, so varied in 

 composition and often so well defined, the condensed meta- 

 phosphates, the molybdates, &c. Certain anhydrides, which 

 are totally wanting in oxy hydroxides, are represented by well- 

 defined salts, as in the case of chromic acid, of which the bi- 

 and tri-chromates, R 2 Cr 2 7 and R2Cr 3 Oi , are well known. 

 Metallic hydroxides also behave like the acid hydroxides, 

 thus : — 



1. Normal Plumbous Hydroxide should be Pb(OH) 2 , but 

 as ordinarily produced it is already partially dehydrated. 

 Dried at a low temperature, it has the formula H 2 Pb 2 3 

 (Tiinnermann, Schaffner, &c.) or H 2 Pb 3 4 , in the form of 

 minute crystalline grains (Payen). At 100° it loses all its 

 water. 



2. Hydrates of Copper. — Cupric hydrate, Cu(OH) 2 , is the 

 bluish precipitate obtained on adding very dilute alkalies to 

 solutions of cupric salts. Heated in water, this hydroxide 

 dehydrates partially and becomes black. It then has the 

 following composition : — 



Dried over sulphuric acid . . H 2 Cu 3 4 (Harms). 

 „ at 100° H 2 Cu 6 7 (Rose). 



Normal cuprous hydrate would have the minimum formula 

 Cu 2 (OH) 2 . On partial dehydration it gives H 2 (Cu 2 ) 8 9 

 (Millon and Commaille), and H 2 (Cu 2 ) 4 5 (Mitscherlich) . 



3. Hydrates of Thorium. — One of the known hydrates, 

 dried at 100°, corresponds to H 4 Th0 4 ; another, obtained by a 

 different method, and dried at the same temperature, cor- 

 responds to H 2 Th 4 9 (Clere). 



4. Manganic Hydrates. — Several hydroxides, formed under 

 various conditions, correspond to the peroxide Mn0 2 , and are 

 as follows:— H 2 MnO s , H 2 Mn 2 4 , H 2 Mn 3 7 , H 2 Mn 4 9 , and 

 H 4 Mn 3 8 . All these dehydrate, when heated, forming Mn0 2 . 



