ELECTRIFICATION OF AIR, VAPOUR OF WATER, AND OTHER GASES. 189 



further experiments to find the greatest electric density which we can measure in 

 air or other gases, 



8. LENAUD'S important discovery of very strong electric effects produced by 

 drops of water falling on a hard surface, gave us a very convenient method for 

 obtaining a steady and strong negative electrification of air, which we used in 

 30-32 for preliminary efforts for finding a good and convenient form of electric filter 

 to be used in further investigations on the electrification and diselectrification of air. 



9. In testing the efficiencies of the electric filters used in method (3), we at first 

 used the filter described in our paper on "Diselectrification of Air," 'Proc. Roy 

 Soc.,' vol. 57, and which consisted of twelve discs of brass wire cloth, fixed in a 

 short metal pipe, supported in a paraffin tunnel. This filter was joined to the 

 insulated quadrants of a quadrant electrometer, and electrified air was sucked through 

 it ( 25) till a convenient deflection was obtained. Then the filter to be tested was 

 connected to the sheath of the electrometer, and so placed that the electrified air 

 passed through the tested filter before it passed through the filter attached to the 

 electrometer ( G8). In this way, by drawing equal quantities of electrified air, as 

 nearly as may be equally electrified, through the different tested filters, a comparison 

 of their relative diselectrifying powers was obtained. For example, <1 being the 

 deflection obtained when no tested filter was used, and d lt d* . . . d, t when the tested 

 filters were successively used, then the relative diselectrifying powers of the filters 



would be "~v~\ " ~T~^> ... -, ", if the primary electrifications were equal. 



10. In other sets of experiments ( 32, 69) we successively joined each separate 

 tested filter to the insulated quadrants of the electrometer, and sucked approximately 

 equal quantities of electrified air through them. The diselectrifying efficiencies of 

 the filters were now approximately in simple proportion to the final readings on the 

 electrometer. 



11. But none of these methods gave us a means of determining the absolute dis- 

 electrifying power of any filter without realizing an equality of primary electrification 

 of the air in different experiments. We therefore, a long time later, used two 

 insulated filters and two electrometers, as described in 55 and fig. 7. Let the filters 

 be called AB and A'B' and their diselectrifying powers n and n'. In a first experiment 

 the electrified air was sucked through AB and A'B' in immediate succession, and in 

 a second experiment the electrified air was sucked through A'B' and AB. 



12. On the assumption that the two filters took out the same 'proportions 

 (respectively n or n) of the electricity of the electrified air entering them in the two 

 experiments, we get the following equations : 



In the first experiment 



Let Q = total quantity of electricity in air entering, 



q l = total quantity of electricity taken out by filter AB, 

 q z = total quantity of electricity taken out by filter A'B'. 



