90 Prof. Louis Henry on the Polymerization 



The chromates are analogous to the sulphates both in their 

 general composition and crystallographical properties. One 

 would expect, therefore, that the difference between Cr0 2 

 and Cr0 2 Cl 2 would be similar to that between S0 2 and 

 S0 2 C1 2 . Now S0 3 boils at 46° and S0 2 C1 2 at 77°, whilst 

 Cr0 2 Cl 2 is a liquid boiling at 118°; and from analogy we 

 should expect Cr0 3 to be also a liquid still more volatile, and 

 boiling even below 100°, whereas Cr0 3 is a solid, fusible 

 only at about 300°. The oxides, in fact, instead of being 

 gaseous at the ordinary temperature, or at least very volatile 

 liquids, as analogy would have led us to expect, are generally 

 solid, and frequently almost infusible. What is the expla- 

 nation of this fact, so strange in its nature and so important 

 from its generality ? I regard no fact, however strange, as 

 abnormal or exceptional. In my opinion anomalies and ex- 

 ceptions have no real existence. They depend most probably, 

 as already stated, on our ignorance of the consequences, under 

 certain conditions, of a general known cause. 



The Oxides are not Moleeularly Comparable to the Chlorides. 

 Polymerization of the Oxides. — The chlorides and oxides are 

 almost universally indicated by formulae which represent these 

 bodies as moleeularly comparable. But are the oxides really 

 comparable with the chlorides of the same elements ? Is there 

 a real analogy between them, or is this analogy merely arti- 

 ficial, and dependent solely on the notation employed ? These 

 are questions which require examination. The chlorides being 

 compounds which are frequently volatile without decomposi- 

 tion, the formulae assigned to them have usually been deduced 

 from the determination of their vapour-density; so that these 

 formulae really represent, both in weight and size, the molecules 

 of these bodies, at least in the gaseous state. This is not the 

 case, however, with the metallic oxides. Being fixed, or 

 volatile only at the very highest temperatures, or else decom- 

 posing; under the action of heat, their vapour-densities are 

 unknown, and cannot be determined. The formulas univer- 

 sally attributed to the metallic oxides depend solely on ana- 

 lytical determinations. These formulae are therefore simply 

 empirical, and merely represent the relations between the 

 weights of the constituent elements, but are perfectly silent 

 as regards the absolute quantities. 



The formulae of the oxides have thus their origin, and rest 

 on an entirely different basis from the formulae of the chlorides. 

 There is no authority for attributing to both the same value 

 and signification. The relation between the oxides and chlo- 

 rides, as represented by the usual formulae, is purely artificial. 



