Long Waves emitted bj/ the Quartz Mercury Lamp. 691 



a 14*66 mm. thick layer of quartz transmitted 46" 6 per cent. 

 of the isolated radiation, when the mercury-lamp served as 

 source o£ light ; and only 21*7 per cent, when the Welsbach 

 mantle was adopted as radiation-source. 



This table shows, for a large number of substances, the 

 transmission of long-waved radiation isolated by means of 

 quartz-lenses. Both sources of light were in use, J) 1 being 

 the transmission observed with the Welsbach mantle, D 2 with 

 the mercury-lamp as radiator. We have, moreover (under 

 D a ), exhibited, for the same substances, the transmission of 

 the radiation of the mercury-lamp filtered by a 2'0 mm. 

 thick layer of amorphous quartz. It could be assumed from 

 the beginning that the observed radiation cf the mercury- 

 lamp consisted of two parts, one of which emanated from the 

 hot quartz-walls of the tube, the other from the mercury 

 vapour itself. For the separation of the latter part we 

 at first deemed a filter of melted quartz most suitable. 

 Later on, we found that a ray-filter of black cardboard 

 proved still more efficient for the isolation of the long- waved 

 radiation emerging irom the mercury vapour. In the last 

 column of our table (D 4 ) we have therefore exhibited the 

 results of the measurements on transmission, obtained after 

 substituting a filter of black cardboard, 0*i38 mm. thick, for 

 the amorphous quartz. 



By reference to the table it will immediately be seen that 

 for all substances the values D 1 , D 2) D s , and D 4 form an 

 ascending series. So far as substances are concerned w^hose 

 region of absorption is known to be situated at shorter wave- 

 lengths (as quartz, fluorite, rock-salt, and sylvine), this 

 course indicates an increase of the mean wave-lengths of the 

 corresponding radiations. We must therefore attribute a 

 greater mean wave-length to this radiation of the mercury- 

 lamp than to that emitted by the Welsbach mantle ; we must 

 further assume a greater mean wave-length for the radiation 

 of the mercury-lamp filtered by black cardboard, than for 

 that which passed through amorphous quartz. This assump- 

 tion is in a still higher degree justified by the behaviour of 

 black paper and black cardboard, because in such media, in 

 which the principal loss of energy is due to diffuse dissipation, 

 the transmission must strongly increase with growing wave- 

 length. The rise of the mean wave-length, which the radia- 

 tion of the mercury-lamp shows after the introduction of the 

 radiation-filters employed, is according to our opinion due to 

 the fact that the short-waved radiation of the quartz-tube 

 (which is nearly of the same quality with that of the Welsbach 

 mantle) is much more strongly absorbed by these filters than 



