﻿of the Atmosphere and Stir/ace of the Earth. 855 



earth. In all these cases, an electroscope placed at some 

 little distance from the cload gave indications of a negative 

 charge being present in the air. This could be detected by 

 the normal positive charge indicated by the electroscope 

 becoming smaller, or if the distance was less than 50 metres 

 the charge would be reversed and potentials up to 200 volts 

 per metre registered. 



In the course of these experiments, it was noticed that if 

 the dust fell upon the electroscope the effect was smaller, 

 as though the dust had imparted a charge of a character 

 different from that given by the air, and further investiga- 

 tion showed this to be the case. The charge upon the dust 

 was always positive. This was easily shown by taking a 

 conical net of fine wire gauze, drawing it rapidly through 

 the cloud so as to catch some of the dust raised by the plough, 

 and then putting it in communication with the electroscope, 

 when a very strong charge of positive electricity was shown, 

 and on some occasions small sparks were drawn from the net. 



The negative charge indicated by the electroscope during 

 a dust storm must thus reside in the air itself, and not in the 

 dust particles caught up by the wind. By way of contrast, 

 it may be noted that when steam suffers condensation in the 

 open air, it gives to the air a positive charge w r hich persists 

 after the cloud has disappeared. The general electrification 

 produced by raising dust composed of different materials has 

 been investigated, and wall form the subject of a future 

 paper. 



Note added April 3rd. 



During a journey from South Africa to Europe obser- 

 vations were taken as occasion offered, and the following may 

 be of interest. 



On March 7th the normal electrification of the atmosphere 

 near Cairo was positive, and at 7 a.m. of the order of 150 volts 

 per metre. On following in the track of a caravan of camels 

 near the Pyramids the potential varied from zero to 50 volts 

 per metre negative. On March 14th, at Pozzuoli near Naples, 

 the potential gradient at sea-level was 80 volts per metre and 

 positive. On ascending to the summit of the Solfatara, the 

 potential fell to zero owing to the action of the steam-jets 

 issuing from the volcano, but on stirring up the fine volcanic 

 dust lying round the vents, a cloud was raised and this caused 

 a negative potential gradient reaching to 100 volts per metre. 



