FORMATION OF SMALL CLEAR SPACES IN DUSTY AIR. 253 



heated, and the box was filled with dust. Examined with the magnifying glass 

 in the usual way, the dark plane was found to be well marked and the repulsion 

 going on as usual, but not the slightest sign of electrification showed itself at 

 the electroscope. No signs of electricity having shown themselves at the hot 

 surface, it was sought for in the ascending current. This was done by first 

 removing the insulated heater and putting in its place the platinum wire, to get 

 a more intense effect from the high temperature. Over the wire was placed a 

 large insulated flat-shaped conductor for the dust to deposit itself upon. The 

 conductor was then connected to the electroscope, the box filled with dust, and 

 the electric current turned on to heat the wire. The leaves of the electroscope, 

 however, remained close together, so that the dust deposited on the conductor 

 could not have been charged with electricity. 



It may be objected to these experiments that the electroscope used was not 

 sensitive enough for the purpose, and that if a more sensitive instrument had 

 been employed, signs of electricity might have been obtained. It is quite 

 possible that another instrument might have shown signs of electrical disturb- 

 ance, but I think that if electricity was the cause of these phenomena, and was 

 sufficiently strong to repel the particles and to cause them to adhere to bodies, 

 it would be quite powerful enough to separate the leaves of the electroscope. 

 Any electrification less than would affect the leaves would only be a secondary 

 matter, and could not be the cause of the phenomena. Another reason for 

 supposing that electricity has little to do with these effects is that the dust tends 

 to settle only on cold surfaces. 



Experiments were now made to see what the effect is of electrifying the hot 

 surface. The small cylindrical conductor was heated, placed in the box, and 

 connected with the electroscope. A Leyden jar charged very slightly, but 

 enough to cause the full divergence of the leaves of the electroscope, was then 

 connected with the apparatus, and the effect on the dust surrounding the electri- 

 fied conductor noted. While the body was hot enough to cause a well-marked 

 dark plane, there was not the slightest effect produced by the electricity, though 

 the leaves of the electroscope were wide apart, and showed that the hot surface 

 had a decided charge. The electroscope was then removed, and a much higher 

 charge given to the conductor. This time an effect was evident, but it was 

 difficult to say what was taking place. The general appearance of the air round 

 the hot conductor had quite changed. The sharp outline of the clear space 

 round it was destroyed, and the dark plane over it had lost its clear and sharp 

 outline, and had become much thicker, though not so dark, as before. All 

 round the conductor there seemed to rage miniature storms, and the particles 

 had much the appearance as if they were seen all out of focus. This effect was 

 produced by either positive or negative charge. 



To find out what was taking place in the air round the electrified body, I 



