694 Long Waves emitted by the Quartz Mercury Lamp. 



maximum at 30 is more conspicuous than in the otlier curves. 

 But even in this curve an accurate determination of these 

 points is very difficult. The assumption is certainly justi- 

 fiable, that the radiation filtered through black cardboard 

 contains a greater amount of this long- waved part than it 

 does after purification by the quartz-filter. We had already 

 deduced this fact from the results of the absorption-table. 



It is still an open question, whether this long-waved 

 radiation consists of several nearly homogeneous kinds of 

 rays of different wave-length — as would be expected upon 

 the assumption of a luminiferous radiation of mercury vapour 

 — or whether it is a continuous radiation, covering a larger 

 spectral region, such as thermo-radiators mostly possess. The 

 results of the interferometer measurements are unable to 

 warrant us in settling this question. But we can safely 

 deduce from our observations, that a large part of this radia- 

 tion emerging from the quartz-mercury lamp possesses a mean 

 wave-length of about 30 x 2 x 523 fi = 314 /u, or nearly ^ mm. 



With a view to confirming our supposition, that this 

 extremely long-waved radiation originates in the mercury 

 vapour itself and not in the hot quartz -tube of the lamp, we 

 cite the following reflection : — As the intensity of the radia- 

 tion from a black body diminishes with the 4th power in the 

 region of great wave-lengths, amorphous quartz (which at 

 \= 100 /n behaves nearly like a black body) might at the 

 threefold wave-length send forth at most the 81st part of 

 the energy it emits at 100 fi. But at the relatively low 

 temperature of the quartz-mantle such a feeble radiation 

 would not be discernible. We could, moreover, show by 

 experiment that the observed long-waved radiation came 

 from the mercury vapour itself. The intensity of radiation 

 was measured shortly before and after the break of the 

 current of the lamp. On introduction of the cardboard-filter 

 the observed intensity of radiation fell, immediately after the 

 interruption of the current, to about 30 per cent, of the initial 

 value, and then slowly diminished more and more. The 

 same experiment without cardboard filter only produced a 

 decrease of radiation of about 30 per cent, after the inter- 

 ruption of the lamp-current. We have, lastly, investigated 

 the radiation we obtained by substituting for the quartz- 

 mercury lamp a piece of amorphous quartz, heated by a 

 Bunsen-flame. This radiation proved to be even of a some- 

 what smaller wave-length than that emitted by the Welsbach 

 mantle under equal conditions. Less than two per cent, of 

 it passed through black cardboard, and only ten per cent, 

 through 2 mm. of amorphous quartz. 



