November 27, 1902] 



NA TURE 



83 



regards the mantle's luminosity as an ordinary high 

 temperature effect ; as showing how the phenomena 

 are accounted for by this explanation, we may quote 

 the view put forward by Mr. J. Swinburne {Journal of 

 the Inst. Elect. Eng., vol. xxvii. p. 161). Mr. Swinburne 

 will have nothing to do with selective emissivity, but 

 states that "all bodies" (presumably solid bodies) "at 

 the same temperature give out light of the same colour." 

 The Bunsen flame, he argues, in which the mantle is 

 immersed, is extremely hot, and the mantle's luminosity 

 is due to its very nearly attaining this temperature. A 

 bad radiator (such as thoria)will reach the same temper- 

 ature as the flame, but as it radiates so little energy will 

 give but little light; what light it does give, however, 

 will be of high luminous efficiency. A good radiator 

 (such as ceria) will radiate energy so fast that it will not 

 attain anything like the flame's temperature. It is, 

 therefore, only necessary to add sufficient ceria to the 

 thoria to increase the emissivity enough to get a good 

 quantity of radiated energy, but not enough to lower 

 the temperature unduly, in order to get a composition 

 giving a brilliantly luminous mantle. This explanation 

 does not appear to us sufficient, especially when one con- 

 siders that it is polished, and not white, bodies which are 

 bad radiators, so that if it is legitimate to argue from 

 their behaviour at low temperature, thoria would be 

 expected to be but little inferior as a radiator to ceria or 

 even carbon. Also there seems some reason to think 

 that selective emission is more probably the rule than 

 the exception (see, for example, the work of Nichols and 

 Blaker, published in the Physical Review). 



Le Chatelier and also Nernst (Joe. cit.), arrive at the 

 same final result as Mr. Swinburne — namely, that the 

 mantle is so bright because it more nearly approaches 

 the temperature of the flame than any other body similarly 

 placed — but by a different argument. The experiments 

 which they made led them to conclude that the emissivity of 

 the mantle is poorin the region of the red rays ; hence there 

 is little energy lost in non-luminous radiations, and the 

 mantle can in consequence come up to the high temper- 

 ature of the flame, at which it begins to radiate well, 

 especially in the region from the green to the violet. The 

 selective emissivity of the mantle material has therefore 

 a double effect ; it increases the luminosity at a given 

 (high) temperature, and it enables the mantle to attain a 

 higher temperature than a black body, because the total 

 loss of energy by radiation is diminished. Bunte, on 

 the other hand, claims that the assumption of selective 

 emissivity is unnecessary, and that the mantle is at a 

 higher temperature than the flame {Berichte Deut. Chem. 

 Ges., 1898, i. 5). This view is supported by experiments 

 he performed, m which different substances were raised 

 to incandescence in pairs in the inside of an electrically 

 heated tube ; no appreciable difference could be observed 

 in the light given by carbon, thoria, ceria or the material 

 of the mantles. It remains to be explained how the 

 temperature of the mantle can be higher than that of 

 the flame. This is due, he and Killing suggest, to the 

 catalytic action of the ceria, which, by oscillating between 

 a low and high state of oxidation, increases the rate of 

 combustion at the mantle surface and so raises its 

 temperature. The thoria is necessary, according to 

 Killing, to give a large surface over which the ceria 

 molecules are spread : and Bunte suggests that it also 

 acts as an insulator between the ceria molecules, enabling 

 them to maintain the high temperature that their catalytic 

 action produces. 



Obviously, the simplest method of testing the accuracy 

 of some of these different hypotheses is to measure the 

 temperatures of mantles of different composition. An 

 attempt to do this has been made quite recently by Messrs. 

 White, Russell and Traver {loc. cit.). The temperatures 

 were measured by means of small thermocouples, and 

 (by making measurements with couples of different sizes 



NO. 1726, VOL. 67] 



and so obtaining data for extrapolation) they claim to 

 have arrived at a method giving with considerable 

 certainty the temperatures of flame and mantle. Even 

 if the accuracy of the absolute values thus obtained be 

 impugned, the relative results are not so subject to the 

 same objections. These experimenters find that the 

 temperature of the mantles and flame is from 1500° C. to 

 1700' C. ; that the mantle is at a slightly lower temper- 

 ature than the flame and at very nearly the same 

 temperature whatever its composition ; and, especially, 

 that a pure thoria mantle is at a slightly higher temper- 

 ature than one of thoria and ceria. Some actual results 

 illustrating these points may be quoted from their 

 paper : — 



Composition of 

 Mantle. 

 Per cent 

 100 thoria 



Temperature of Temperature of Candle-power 

 Mantle. Il.mic per sq. in. 



c. c. 



99'5 thoria ) 



1560 ... 1630 1 ... y$ 



1S20 ... 1650° ... 54-0 

 & 05 ceria J 3 3 J * 



The mantles used are said to have been identical 

 in every respect except in their chemical composition. 

 The differences in temperature are not very great, but, 

 such as they are, they do not harmonise with the theory 

 of le Chatelier and Nernst, since they show the thoria 

 mantle to be the hotter ; at the same time, they support 

 this theory as against that of Bunte by showing the 

 mantle to be at a lower temperature than the flame. 

 The results also support the views of Mr. Swinburne, 

 which require that the order of the temperature should 

 be the same as that observed. In some other experi- 

 ments, the results were less conclusive, the illumination 

 varying from 2.'5 to 48 candles with practically no tem- 

 perature difference. Mantles with a high percentage of 

 ceria were not tested. The authors themselves conclude 

 that the illumination is to a greater degree a specific 

 function of the material than it is of the temperature, 

 and that the particular thoria-ceria mixture is a solid 

 solution capable of transforming the heat of the flame 

 into light more economically than any other substance 

 yet known. 



If this explanation is to be accepted, the mechanism 

 by which this transformation, is effected remains to be 

 explained. In that part of the paper by Prof. H. E. 

 Armstrong {loc. cit.) which deals with the question of 

 luminosity, we find a suggestion as to what this mechanism 

 is. Prof. Armstrong's paper is of a comprehensive and 

 far-reaching character, dealing with many things besides 

 luminosity in general and that of the mantle in particular, 

 but it is only its bearing on these questions that we can 

 consider here. Prof. Armstrong thinks that "luminosity 

 and line-spectra are the expressions— the visible signs — 

 of the changes attending the formation of molecules from 

 their atoms, or, speaking generally, that they are con- 

 sequences oj chemical changes!' Applying this to the 

 Welsbach mantle, after referring to Bunte's hypothesis, 

 he says, "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 oscillatory or recurrent oxidation.' 1 

 This process, then, gives direct birth to the luminous 

 radiations and accounts for the high efficiency of incan- 

 descent oxides generally, such as the lime and zirconia 

 light and the Nernst glower. A somewhat similar con- 

 clusion is arrived at by Dr. Auer von Welsbach {loc. cit.), 

 who considers that the ceria when in one or other state 

 of oxidation can form a compound with the thoria :. 

 hence " if reduction takes place, there is also decomposi- 

 tion, and if oxidation, there is recombination of these 

 elements ; these reactions may go on several million 

 times a second, and molecular shocks are produced' 

 which give rise to luminous oscillations of the ether, and 



