L .689 ] 



LXXX. On Extremely Long Waves, emitted by the Quartz 

 Mercury Lamp. By H. Rubens and 0. von Baeyer*. 



AN advance in the spectrum towards the side of the long 

 waves is extremely difficult, while using pure thermo- 

 radiators. If the source o£ heat does not possess selective 

 qualities, the intensity of radiation in the long-waved spectrum 

 diminishes with the 4th power of the wave-length. It is 

 true this intensity of radiation grows in proportion to the 

 temperature of the source ; but in a much higher degree 

 (with the 4th power of the absolute temperature) the total 

 energy of the radiant body augments, from which the 

 desired part of radiation must be sorted out by certain 

 processes. An increase of the temperature of the source of 

 light will, therefore, in many cases scarcely involve an ad- 

 vantage for the present purpose. In the long-waved spectrum 

 the Welsbach mantle has proved the most advantageous 

 source of heat of purely thermoradiant character, because of 

 its very favourable selective qualities. But even here, no 

 rays of much greater wave-length than 108 \x have been 

 obtained. 



This paper gives a description of experiments undertaken 

 with a view to enlarge the knowledge of the infra-red 

 spectral region, by employment of sources of light, from 

 which the radiation is emitted by incandescent gases. As 

 far as pure radiation of temperature is concerned, such light- 

 sources are selective in the highest degree. Besides, the 

 possibility of an existing long-waved infra-red luminescence 

 radiation must here be considered. 



Our arrangement of apparatus is identical with that used 

 recently by Ii. W. Wood and one of us for the isolation of 

 long-waved raysf. It is founded on the use of quartz-lenses, 

 which, because of the extreme difference of the index of 

 refraction for heat-rays on both sides of the region of absorp- 

 tion in quartz (1*50 to 2'14), can be so adjusted as to con- 

 centrate the emitted long- waved radiation on a given 

 diaphragm, while the ordinary heat-waves are dispersed. 

 Our method is further founded on the selective absorption of 

 quartz and on the effect of certain central screens. For all 

 details concerning the apparatus and the method reference 

 must be made to the above-cited paper. 



The first sources of light we now used were strong leyden- 

 jar sparks between electrodes of zinc, cadmium, aluminium, 

 iron, platinum, and bismuth ; the sparks were produced by a 



* Communicated bv the Authors. 



t H. Rubens & R.W. Wood, Phil. Mag. Fob. 1011. 



