of the Metallic Oxides. Ill 



with every combination of heterogeneous bodies ; and this dis- 

 engagement of heat may sometimes be considerable, as in the 

 case of the internal combustion (spontaneous in the com- 

 plete sense of the word), which certain metallic oxides 

 undergo on heating (oxydes cuits of Chevreul). 



Th6 quantity of heat disengaged on the formation of an 

 oxide RO* in the direct way has a double origin, viz. the heat 

 of combination of the elements and the heat of polymeriza- 

 tion ; so that the direct formation of a chloride is not at all 

 comparable with that of an oxide. A comparison, therefore, 

 of the phenomena of combination, from the physical point of 

 view, shows that the heat evolved is an absolute measure of 

 the difference of the affinities between the elements of the 

 compound. That being so, we should conclude that the dif- 

 ference between the affinity of oxygen and chlorine for the 

 metals is in reality greater than the thermal phenomena, as 

 determined directly, would lead us to assume. 



Action of Heat on the Oxides — their Classification. 



The oxides may be divided into various groups, according 

 to the changes which they undergo on heating. 



A. Normal or Regular Oxides, which correspond molecu- 

 larly to the chlorides. These oxides are few in number, e. a. 

 S0 2 , C0 2 , NO, N 2 0, CO, &c. 



B. Polymerized Oxides, the molecules of which are made 

 up of n molecules of the normal oxide united together. This 

 group includes most oxides, and may be divided into several 

 classes according to the action of heat thereon ; viz. : — 



(1) Volatile oxides, which are totally depolymerized on heat- 

 ing, and in the state of vapour are converted into the normal 

 oxide. The number of these oxides is limited, and they are of 

 two kinds : — 



(a) Oxides which are completely depolymerized on volatiliza- 

 tion, yielding a vapour of normal density, e. g. S0 3 , 

 Os0 4 , and probably Ru0 4 ; also various organic oxides, 

 such as methylene oxide (CH 2 0) n , metaldehyde 

 (CH 3 . CHO)„, and aldehydes and acetones in general. 

 On condensation the original polymeric oxides are 

 re-formed. 



(b) Oxides which are depolymerized very imperfectly at the 

 moment of volatilization, and only completed progres- 

 sively as the temperature rises. The vapour-density of 

 these bodies, therefore, gradually diminishes with in- 

 crease in temperature up to a certain point, beyond 

 which it becomes constant, and corresponds to that of 

 the normal oxide, e. g. N 2 4 , fatty acids, and paral- 



