238 Proceedings of the Royal Society of Edinburgh. [Sess. 
soon be covered with a film of water, which we have found stops all 
discharge of nuclei ; or the absence of the large nuclei when amalgamated 
zinc was used might have been due to a perfect coating of mercury. 
Dust in an Electric Field. 
A few studies were made of the effect of an electric field on dust 
particles, to see how the dust moved under the strain. For these experi- 
ments a large glass beaker 12 cm. diameter and 25 cm. deep was used. 
The top was covered with a piece of wood with a hole in the centre 
through which projected an insulated metal rod 6 mm. diameter. This 
rod was connected with a gold-leaf electroscope and kept charged by 
a very small friction electrical machine, which supplied either + or — 
electricity. The machine had a cylinder of only 1*5 cm. diameter. The 
limit of the charge was regulated by the electroscope, which caused a 
discharge when the potential was high enough to cause the leaves to 
diverge to an angle of about 45°, when they came in contact with an 
earthed conductor. For dust, the products of combustion of a small 
piece of magnesium ribbon were used. The ribbon was burned under 
the inverted beaker, and when cold the wooden cover was slipped 
underneath and turned with the cover upwards, and the insulated 
metal rod passed through the hole into the interior among the dust. 
Magnesia was used because the size of the particles' enables us to 
follow their movements more readily than can be done with very 
small particles. 
On first charging the rod there is very little perceptible effect, but 
after a minute or so flocculation is seen to be rapidly taking place, and on 
examining the rod and the inside surface of the beaker these are found 
to be covered with projecting threads composed of dust particles. Some 
of the threads may be a centimetre or more in length. Between the 
threads on the rod and those on the glass will be seen a number of threads 
in rapid movement going between the projecting threads on the rod 
and the sides and falling at the same time. So far as could be observed, 
there was little action at first, but very soon dust particles were seen 
to be attracted, and adhered to the rod. These particles, owing to their 
projecting from the rod, acquire a higher electric density than the rod : 
this causes other particles to be attracted and attached to them, causing 
an increase in their length, so increasing the charge at their points, and 
thus quickening up the activity. These threads all point radially and 
horizontally from both surfaces, and they seem to grow till their charge 
causes them to break away and dart to a projecting thread on the 
