1894.] On the Electrification of Air. 85 



us* as probable that in ordinary natural atmospheric conditions, the 

 air for some considerable height above the earth's surface is electri- 

 fied,f and that the incessant variations of electrostatic force which he 

 had observed, minute after minute, during calms and light winds, and 

 often under a cloudless sky, were due to motions of large quantities 

 of positively or negatively electrified air in the immediate neigh- 

 bourhood of the place of observation. 



2. It was proved]; by observations in the Old College of Glasgow 

 University that the air was in general negatively electrified, not only 

 indoors, within the old lecture room of Natural Philosophy, but 

 also in the out-of-doors space of the College Court, open to the 

 sky though closed around with high buildings, and between it and 

 the top of the College Tower. The Old College was in a somewhat 

 low situation, surrounded by a densely crowded part of a great city. 

 In the new University buildings, crowning a hill on the western 

 boundary of Glasgow, similar phenomena, though with less general 



with respect to the ground It is not clearly proven that a pure gas, 



rarefied or not, can receive and convey a charge. If we imagine a charged drop of 

 water suspended in air and evaporating, it follows that, unless the charge be carried 

 off in the vapour, the potential of the drop would rise steadily as its surface dimin- 

 ished, and would become infinite as the drop disappeared, unless the charge were 

 dissipated before the complete drying up of the drop by dispersion of the drop itself, 

 or conveyance of electricity by its vapour. The charge would certainly require to 

 pass somewhere, and might leave the air and vapour charged." 



It is quite clear that " must " ought to be substituted for " might " in this last 

 line. Thus the vagueness and doubts expressed in the first part of the quoted 

 statement are annulled by the last three sentences of it. 



* " Even in fair weather the intensity of the electric force in the air near the 

 earth's surface is perpetually fluctuating. The speaker had often observed it, 

 especially during calms or very light breezes from the east, varying from 40 

 Daniell's elements per foot to three or four times that amount during a few minutes, 

 .and returning again as rapidly to the lower amount. More frequently he had 

 observed variations from about 30 to about 40, and back again, recurring in uncer- 

 tain periods of perhaps about two minutes. These gradual variations cannot but 

 be produced by electrified masses of air or cloud, floating by the locality of observa- 

 tion." Lord Kelvin's ' Electrostatics and Magnetism,' art. xvi, 282. 



t The out-of-doors air potential, as tested by a portable electrometer in an open 

 place, or even by a water-dropping nozzle outside, two or three feet from the walls 

 of the lecture room, was generally on these occasions positive, and the earth's 

 surface itself therefore, of course, negative the common fair weather condition 

 which I am forced to conclude is due to a paramount influence of positive elec- 

 tricity in higher regions of the air, notwithstanding the negative electricity of the 

 air in the lower stratum near the earth's surface. On the two or three occasions 

 when the in-door atmospheric electricity was found positive, and, therefore, the 

 surface of the floor, walls and ceiling negative, the potential outside was certainly 

 positive, and the earth's surface out-of-doors negative, as usual in fine weather." 

 Ibid., 300. 



$ Ibid., Q. 2, 283. 



Ibid., 296300. 



