112 Prof. Louis Henry on the Polymerization 



dehyde. The vapour-density of the last corresponds to 

 (C 2 H 4 0)3 at low temperatures, and at higher tempera- 

 tures to (C 2 H 4 0), i. e. the normal aldehyde. 



(2) Volatile oxides, which are only depolymerized imper- 

 fectly, at least at the temperatures to which they have been sub- 

 jected ; e. g. (As 2 3 ) w , the gaseous molecule of which is 



(As 2 3 ) 2 *. 

 Lactide, or the dioxide of the radical C 3 H 4 iv (orCH 3 . CH.C=), 



I 

 is perfectly analogous to arsenious oxide, its molecule in the 

 state of vapour corresponding to (C 3 H 4 2 ) 2 . Ethylene dioxide 

 (C 2 H 4 0) w probably belongs to this class, as in the state of 

 vapour its molecule = (C 2 H 4 0) 2 . 



(3) Oxides which are not capable of depolymerization. Most 

 inorganic oxides belong to this group ; also the oxides of 

 certain organic radicals. 



They are of two kinds : — 



(a) Oxides which are absolutely indepolymerizable and fixed, 

 as in the case of many metallic oxides which are not 

 decomposed by heat ; e. g. Si0 2 , B 2 3 , Sn0 2 , ZnO, 

 MgO, Fe 2 3 , Cr 2 3 , A1 2 3 , PbO, CuO, Cu 2 0, &c. 



(b) Oxides which are indepolymerizable, because on heating 

 they are decomposed either into their elements or into new 

 compounds, as in the case of several metallic oxides ; 

 e. g. HgO, Ag 2 0, Hg 2 0, Cr0 3 , Mn0 2 , &c, and also of 

 certain organic oxides, as glycollide (C 2 H 2 2 ) w , which 

 carbonizes on heating. 



The above classification is conveniently represented in the 

 following table (p. 113). 



It follows from all these facts and considerations that there 

 are large and important lacunae in the series of oxygenated 

 compounds of Mineral Chemistry. Shall we ever be able to 

 obtain the whole series of normal oxides, of which we have at 

 present so few examples ? I know not. But to produce them 

 we should doubtless have to work under conditions totally 

 different from those which are usually employed. Our actual 

 methods of preparation are precisely those under which these 

 unknown bodies polymerize, and thus escape us. If it be 

 rash to hope to obtain these bodies in the near future, it would 

 be still more so to say that their discovery was impossible. 

 This word is profoundly repugnant to our tendencies and scien- 

 tific habits. When we measure the progress of science since the 

 commencement of the century in a domain so vast as Che- 

 mistry, there are many reasons for not being discouraged. 



* Arsenious oxide volatilizes, as is well known, at about 200° without 

 melting. This temperature is not the boiling-point properly so called, 

 but is really the temperature of depolymerization of the molecule (As a 3 ) n 

 into (As 2 0,) 2 . 



