FORMATION OF SMALL CLEAR SPACES IN DUSTY AIR. 255 



dark plane remains quite clear, but as the temperature falls, a stage is arrived 

 at when the electrical effect overcomes the heat effect, and the particles break 

 in on the dark space and destroy it. 



In making electrical experiments, most of us have noticed the tendency which 

 dust in the room has to settle on the different parts of the electrical apparatus, 

 and to destroy the insulation, and many have noticed the excited and rapid 

 movements of electrified dust. Dr Lodge, in the letter already referred to, 

 remarks on the rapidity with which the dust-box, in his experiments, was cleared 

 of its dust by means of electrified bodies placed inside it. I have made some 

 experiments on this subject, to determine the conditions most favourable for the 

 clearing of air by means of electricity. For these experiments I preferred to 

 use a large glass flask about 30 cm. in diameter. Placing this flask with its 

 mouth downwards, I introduced into it an insulated metal rod, fixed vertically, 

 and passing through the open neck of the flask. If a dense cloudiness was 

 made in the flask with any dust, by preference it was generally made by burn- 

 ing sulphur and adding a little ammonia. After a dense whiteness had been 

 produced, the conductor was electrified. Seen from a distance, no change 

 seemed to have taken place, but on examination it was found that all the dust 

 was deposited on the inside of the flask in a nearly uniform white coating. To 

 enable me to see what was taking place, the inside of the flask was wetted. 

 When the electrification began, the dust could now be seen driven about 

 as by a violent wind, and, after a few turns of the machine, it had disap- 

 peared from the flask. The conditions found most suitable for producing this 

 result quickly were a rapid discharge of the electricity into the dusty air by 

 means of a point or points. If the conductor terminates in a ball inside the 

 flask, the electrification has but little effect. In addition to the conductor 

 terminating in a point, it is also necessary to have near the electrified point 

 surfaces to aid in the rapid electrification of the dust. When the point is 

 surrounded by surfaces the air currents are violent, but if we remove the sur- 

 faces the currents are not nearly so strong. This may be seen by allowing a 

 cloud of dust to rise round a conductor placed in an open space, when but little 

 effect will be observed on electrification. After the dust has been electrified, 

 it ought to be brought near some surface, towards which it may be attracted, 

 otherwise it may lose its charge before meeting a place to deposit itself. 



Experiments have also been made to determine whether the very fine and 

 invisible dust of the atmosphere is also caused to deposit itself when electrified. 

 With this object the large glass flask had an india-rubber stopper fittei to it, 

 through which passed a tube to connect the interior of the flask with an air- 

 pump, to test the condition of the air in the flask by reducing its pressure, 

 while it was kept moist by the presence of water, and to observe whether any 

 cloudy condensation took place after electrification. A conductor insulated in 



VOL. XXXII. PART II. 2 U 



