On the Change of Absorption produced by Fluorescence. 485 



Method 3. Observation of the electricity taken out of air by the 

 electric filter. 



§ 5. Method 1, was used in the experiments described in their 

 communication to the Royal Society of February, 1895, from which 

 it was concluded that air and several other gases tried became 

 electrified by blowing them in bubbles through water, and through 

 solutions of various salts, acids, and alkalis, in water. This conclu- 

 sion was verified for the case of common air and pure water by 

 collecting, into a large reservoir over water, air which had been 

 bubbled through pure water in a U-tube. The electrification of the 

 air thus collected was tested by a water-dropper, taking the same 

 potential as the air at the centre of the reservoir. It was thus 

 proved that the electrification of the air was negative, as was to be 

 expected from the positive electrification which the authors had 

 found on insulated vessels containing water through which air had 

 been bubbled. 



§ 6. Method 2, was used in the first experiments described in the 

 present paper (§§ 16 — 24) which were undertaken for the purpose 

 of determining approximately in absolute measure the total quantity 

 of electricity in a given mass of electrified air ; and particularly for 

 finding the greatest electrification which could be communicated to a 

 large quantity of air by needle points supplied with electricity from 

 an electric machine. The result thus found in § 23, 3*7 X 10~ 4 C.Gr.S. 

 electrostatic, is the greatest electric density (quantity of electricity 

 per cubic centimetre) which the authors have hitherto been able to 

 measure in air electrified by electrified needle points. But by an 

 electrified hydrogen flame a density of 22 x 10~ 4 O.G.S. electrostatic 

 unit was obtained in air (§ 65). 



§ 7. In all the other experiments described in the present paper, 

 Method 3 was used ; but probably the authors must return to Method 

 2 if, in future, they undertake further experiments to find the greatest 

 electric density which they can measure in air or other gases. 



<' On the Change of Absorption produced by Fluorescence." 

 By John Burke, B.A. (Dub.), Berkeley Fellow of the 

 Owens College, Manchester. Communicated by Professor 

 Arthur Schuster, F.R.S. Received June 10, — Read June 

 17, 1897. 



(Abstract.) 



If a body, A, of some fluorescent substance, such as uranium 

 glass, be transmitting light from a similar body, B, which is fluo- 

 rescing, the amount of light transmitted by A from B seems quite 

 different, according as A is fluorescing or not. There appears to- 



