of the Metallic Oxides. 95 



obtained by dehydration of their hydroxides at as low a tem- 

 perature as possible, suddenly enter into vivid incandescence 

 when exposed to a higher temperature, verging on redness. 

 Their properties are thus modified both physically and che- 

 mically; they become denser, harder, and, when coloured, 

 their colour is intensified. They show a greater resistance to 

 the action of chemical agents than in their former condition, 

 some even entirely resisting the action of acids and alkalies. 

 Fe 2 3 , Cr 2 3 , Zr0 2 , &c, are examples. The fact of this vivid 

 incandescence, which is not accompanied by any alteration in 

 the composition of the product, is the manifest proof of an 

 energetic combination of the oxide with itself. When hydro- 

 carbons, such as amylene, terebenthene, &c, are polymerized 

 under the action of certain agents, such as sulphuric acid, 

 and especially boron fluoride, a development of heat takes 

 place which is frequently considerable. Finally, arsenic tri- 

 oxide is a direct proof of accumulation in a single molecule, 

 even in the gaseous state, of several normal molecules corre- 

 sponding to the chloride. Arsenic trichloride being AsCl 3 , 

 the corresponding oxide ought to be As 2 3 . This is in fact 

 the arsenical oxygenated compound which is present in the 

 arsenites of methyl and ethyl {Bull. Soc. Chim. xiv. p. 101), 

 compounds which are volatile without decomposition, and 

 whose formulae, as deduced from vapour-density determina- 

 tions, are (CH 3 ) 3 As0 3 and (C 2 H 5 ) 3 As0 3 . The molecule of 

 arsenious oxide in the gaseous state corresponds not to As 2 3 , 

 but to (As 2 3 ) 2 or As 4 6 , as proved by its vapour-density. It 

 volatilizes at about 200° without fusion ; and there can be no 

 doubt that its molecule in the solid state is some multiple of 

 its gaseous molecule As 4 6 . There is thus established between 

 the methylenic compounds and the arsenious ethers a complete 

 parallel. To this we shall refer again. 



Various Methods of Producing the Oxides. 

 By Dehydration of the Hydroxides. — The various methods of 

 obtaining the oxides from other compounds containing them 

 as radicals also tend to show that they are polymers. Among 

 these methods, one of the most important, on account of its 

 generality, and also the most interesting from our particular 

 point of view, is the dehydration of hydroxides. Under the 

 action of heat the hydroxides, with but few exceptions, lose 

 water, either wholly or partially. But, however this may be, 

 the dehydration is the more complete the higher the tem- 

 perature. This temperature varies according to the nature 

 of the hydroxide. Some of the latter have only an ephemeral 

 existence at the ordinary temperature, and are therefore more 



