104 Prof. H. E. Armstrong. On Chemical Change, [Mar. 13, 



In the discussion on Mr. Campbell Swinton's paper on the luminosity 

 of the rare earths when heated in vacuo by means of cathode rays, 

 read in April, 1899,* I applied this view in explanation of the brilliant 

 luminosity of certain rare earths, and also of the electric arc, suggesting 

 in the latter case that the carbon molecules were dissociated by the 

 passage of the current, and that the re-association of the atoms into 

 molecules gave rise to the intensely luminous effect which is charac- 

 teristic of the arc. 



Bunte has argued, f in the case of the Auer von Welsbach gas 

 mantle, that the high luminosity is not so much due to a specific 

 superior radiating power of the earth used as to the fact that, in 

 virtue of the power which a substance such as ceria possesses of form- 

 ing a peroxide, combustion takes place to a greater extent at a surface 

 on which it is present than on one consisting of a neutral oxide : 

 consequently, the temperature is higher at such a surface. This 

 undoubtedly must be the case ; but I would go further, and regard the 

 chemical changes occurring at the surface as the direct seat, or origin as 

 it were, of the luminosity. Probably, a higher oxide is alternately 

 decomposed and reformed— in other words, the process is one of oscil- 

 latory or recurrent oxidation. Owing to the influence which the oxide 

 exercises as a catatyst, combustion doubtless takes place at its surface 

 more completely than it would at a corresponding neutral surface, and 

 in consequence the temperature developed at the oxide surface is 

 above that of the flame generally. 



[It is noteworthy that besides ceria, which is by far the most effi- 

 cient, only one or two other oxides are effective excitants ; and that 

 cerium dioxide is remarkably stable at high temperatures. — Note, added 

 May 2.] 



A similar explanation may be applied to the incandescence of oxides 

 generally, such as is witnessed, for example, in the lime and zirconia 

 lights. The Nernst lamp probably owes its efficiency to a like cause. 

 It may well be that in the case of the incandescent mantle the maxi- 

 mum effect is produced when only a relatively small proportion of 

 active oxide is present, because at the particular state of dilution the 

 oxide is present in solid solution in a form in which it is most prone 

 to suffer change — that besides being placed under the necessary con- 

 ditions of freedom, it attains to the most suitable degree of molecular 

 complexity, and therefore to its greatest activity, in dilute solid solu- 

 tion just as many substances do in dilute liquid solutions. 



Mr. Campbell Swinton's observations appear to me to be in harmony 



of Frankland's theory, as I understand it, and as appears to be necessary in order 

 to explain the luminous appearance of a hydrogen-oxygen flame burning under 

 pressure, and other cases in which the products of combustion are gaseous." — 

 ' Chem. Soc. Trans.,' 1895, p. 1147. 



* ' Roy. Soc. Proc.,' vol. 65, p. 115. 



f c Deut. Chem. Ges. Ber.,' 1898, p. 5. 



