166 



SCIENCE 



[N. S. Vol. XXXI. No. 7f 



mtim temperature which it withstands for 

 a few moments, as shown in the experi- 

 ments, then the cost of operating incandes- 

 cent lamps could be reduced to nearly a 

 fifth of the present cost. 



It was discovered by Auer von Welsbach 

 that the metal osmium could be made into 

 a filament, though it could not be drawn as 

 a wire. The osmium lamp was the first of 

 the recent trio of metallic filament incan- 

 descent lamps. The tantalum lamp, in 

 which another high melting-point metal 

 replaces the superior but more expensive 

 osmium, has been in use six or eight years. 

 This surpasses the carbon in its action, and 

 on running up to its melting-point it shows 

 still brighter light than carbon. More 

 recently the tungsten filament lamp has 

 started to displace both lamps. At present 

 this is the element which withstands the 

 highest temperature without melting or 

 vaporizing, and on being forced to its high- 

 est efficiency in a lamp you see that it 

 reaches higher luminosity and that there is 

 a similarity to carbon and tantalum in 

 that an enormouslj^ greater efficiency may 

 be produced for a very short time than can 

 be utilized for a suitable length of life. 

 The inherent changes at these tempera- 

 tures, distillation or whatever they are, 

 quickly destroy the lamp. The lamp will 

 burn an appreciable time at an efficiency 

 fifteen times as great as that of the common 

 operating carbon incandescent lamp (at 3 

 watts per candle). In other words, light 

 may be produced for a short time at an 

 energy-cost one fifteenth of common prac- 

 tise, so that there is still a great field for 

 further investigation directed towards 

 merely making stationary those changing 

 conditions which exist in the burning lamp. 



While it is generally true that the light 

 given by a heated body increases very rap- 

 idly with rise of temperature above 600°, 

 the regularity of the phenomenon is com- 



monly over-estimated. A certain simple 

 law covering the relation between the tem- 

 perature and the light emitted, has been 

 found to apply to what we have called a 

 black body. This so-called Stefan-Boltz- 

 mann law states that "the total intensity 

 of emission of a black body is proportional 

 to the fourth power of the absolute tem- 

 perature." There are, however, very few 

 real black bodies in the sense of the law. 

 The total emission from a hole in the wall 

 of a heated sphere has been shown experi- 

 mentally to follow the law rigidly, but most 

 actual forms and sources of illumination 

 do not. Most practical sources of artificial 

 light are more efficient light producers than 

 the simple law requires. This may be said 

 to be due to the fact that these substances 

 have characteristic powers of emitting rela- 

 tively more useful energy as light than 

 energy of longer wave-length (or heat 

 rays). Most substances show a power of 

 selective emission and we might say that 

 an untried substance, heated to a tempera- 

 ture where it should be luminous, could 

 exhibit almost any conceivable light effect. 

 It is still less possible to predetermine the 

 proportionality between luminous and non- 

 luminous emission. A simple illustration 

 will serve to make this clear: if a piece of 

 glass be heated to 600°, it does not emit 

 light. If some powder such as zireonia or 

 thoria be sprinkled upon it, light is emitted 

 and the proportion of light at the same 

 temperature will depend upon the composi- 

 tion of the powder. Coblentz has shown, 

 both for the Auer mantle and for the 

 Nernst glower, that the emission spectra are 

 really series emission bands in that portion 

 of the energy curve which represents the 

 larger part of the emitted energy. This is 

 in the invisible infra-red part, and so the 

 laws which govern the emission at a given 

 temperature depend upon the chemical 

 composition of the radiant source. Sili- 



