1907.] Incandescent lUuminants. 619 



WEEKLY EVENING MEETING, 

 Friday, April 26, 1907. 



The Right Hon. Lord Alverstoxe, G.C.M.G. P.C. M.A. D.C.L. 

 LL.D. F.R.S., Vice-President, in the Chair. 



James Swixburxe, Esq., F.R.S. M.Inst.C.E. M.R.I. 



Incandescent lUuminants. 



[abstract.] 



A little more than twenty years ago, Auer von Welsbach, who was 

 engaged on researches on the rare earths, invented the modern 

 incandescent mantle. His first mantles were made of zirconia and 

 yttrite earth, in the proportion to make a normal zirconate. Shortly 

 afterwards, he found that the best material has a basis of thoria. 

 Pure thoria, which requires care in its preparation, gives very little 

 light, but if a small percentage of a coloured and permanent oxide, 

 such as ceria, is added, it gives good illumination. 



There has been much discussion about the theory of the incan- 

 descent mantle. It has been generally assumed that the temperature 

 of a Bunsen burner is too low for a mantle to give the light it does 

 by simple radiation, unless it is much hotter than the flame. Un- 

 fortunately, the temperature of the flame is generally taken with a 

 thermo-couple, and this gives far too low a reading, as the thermo- 

 couple never reaches the real temperature of the flame. But, 

 admitting that the temperature of the flame is high, it is still urged 

 that the light given by the thoria with a small percentage of ceria is 

 so great that there is something else than mere thermal radiation. 

 It is said that the ceria acts as a catalytic agent, and that it oscillates 

 between two states of oxidation. Ceria does act in somewhat the 

 same way as platinum ; for instance, if a ceria mantle is put on a 

 hghted burner, and the burner turned out, and the gas turned on 

 again, the ceria mantle will glow and will finally hght the gas. It is 

 odd that this is not brought forward by the advocates of the 

 catalysis theory ; but the opponents might urge that zirconia will 

 do the same thing, and the zirconia mantle gives very httle hght. 

 This does not really dispose of the catalytic theory. 



According to the simple radiation theory, the hght depends only 

 on the emissivity, or blackness of the mantle, and its temperature. 

 Its temperature must be lower than the flame, as it must be robbing 



