690 Profs. H. Rubens and 0. von Baeyer on Extremely 



40 cm. inductor, using alternating current for the inner coil. 

 We have, however, in no case succeeded in obtaining a per- 

 ceptible radiation in the observed long-waved spectral region. 

 As little success was gained when we used the electric arc with 

 carbon electrodes, or with Bremer carbons and carbons with 

 iron-salt rilling, if the investigation was limited to tbe electric 

 arc itself. It is true, in both these last-named cases our 

 micro-radiometer always showed small irregular deviations, 

 which undoubtedly were due to long-waved radiation ; but 

 it is not improbable that this radiation is emitted by solid 

 particles in the electric arc. The observed effects were neither 

 regular enough nor sufficiently strong to allow of a closer 

 investigation. 



A comparatively very strong long-waved radiation was, 

 however, obtained with the quartz-mercury lamp, especially at 

 higher consumption of energy *• With a current of 4 amperes 

 on 100 volts, the arc being about 80 mm. long, a deflexion of 

 more than 50 mm. appeared in our micro-radiometer. When 

 the lamp had burned some time this deviation proved so 

 constant that it conld easily be measured down to fractions 

 of a per cent. 



A few preliminary experiments showed us that the observed 

 long- waved radiation of the mercury- lamp must possess a 

 composition essentially different from that of the Welsbach 

 mantle, the mean wave-length of which had, under the same 

 conditions, amounted to 108 fi. We found, for instance, that 



Substance. 



Quartz 



Amorphous quartz 



Fluorite 



Rock-salt 



Syrvine 



Diamond 



Selenium 



Mica 



Glass 



Paraffin 



Hard rubber 



Amber 



Wood 



Black Paper 



Black Cardboard .. 

 Celluloid 



Water 1 



d. 



D v 



mm. 



Per cent. 



417 



121 



200 



12-5 



059 



5-3 



1-29 



05 



2-10 







1-26 



45-3 



0-52 



6-8 



0-055 



16-6 



018 



2-1 



3 03 



570 



0-40 



390 



1-28 



11-2 



ISO 



0-7 



011 



335 



0-38 



21 



0-26 



162 



0019 



— 



0-038 



20 



D 2 . 



Per cent. 



25-4 

 242 

 19-4 



5-7 



36 

 64-5 

 129 

 38-8 



90 

 72-3 

 515 

 16-4 



29 

 524 

 11-7 

 276 

 55-5 

 33-0 



D 3 . 



Per cent. 



51-8 



39-5 

 16-5 

 117 



24-2 

 515 

 211 



82-5 

 58-8 

 32-2 

 10-0 

 78-5 

 29-8 

 38-7 

 60-3 

 38-4 



D 4 . 

 Per cent. 



58-9 

 60-0 

 42 '2 

 22-5 

 167 



550 

 259 

 85-5 

 653 

 348 



79-0 

 367 

 43-5 



627 

 39 8 



* A mercury-amalgam lamp, containing 20 per cent, bismuth and 

 20 per cent, lead on 00 per cent, mercury gave nearly the same results. 



