491 
1910-11.] On some Nuclei of Cloudy Condensation. 
on the surrounding surfaces. But there is a particular I do not remember to 
have seen previously noticed, and that is, that though the electricity can 
clear away the dust it does not always leave the air nucleus-free ; only under 
certain conditions will the electricity leave the air free from nuclei of 
cloudy condensation when it is supersaturated. It is only when the 
potential of the electricity is kept below a certain value that the air is 
made dustless and unable to give any condensation with slight super- 
saturation. If the potential rises above a very small value, then, though the 
dust be deposited, something is produced by the discharge — probably ozone 
and oxides of nitrogen — which when tested in moist air is found to give 
a dense fog with but little supersaturation. This product of the electric 
discharge seems to be in the gaseous state, because after it has been formed 
it cannot be removed by a discharge at a low potential, which it would be 
if the something were in a solid or liquid condition, such as might be 
produced by the disintegration of the discharging point. As the potential 
which gives this gaseous nucleus-producer is very low, all attempts at 
clearing out dust and fog on a large scale, where high potentials must be 
used, only result in getting rid of the solid and liquid particles and 
probably moisture-absorbing ones, and replacing them with gaseous matter, 
which forms nuclei whenever the air is cooled to the dew-point. Further, 
the point discharge not only acts on pure air, producing nuclei, but if there 
are any impurities, such as sulphurous acid, which is always present in town 
air, the density of the condensation is greatly increased ; water vapour has 
also a similar effect, and it is always in abundance when fogs are being 
cleared with electricity. From this it would appear that air that has been 
cleared from fog by electric discharge is still in a condition to give a dense 
condensation when cooled to the dew-point. 
Having ascertained the effect of the point discharge in ordinary air, 
namely, that only very low potentials can be used without producing nuclei, 
and that the potential must be lower for damp air ; further, that it was 
necessary to cleanse the platinum point by keeping it in a flame for a time, 
as an unclean point gave dense condensation with low potentials ; — a little 
salt or iodine, or both, was put in the flask and the point electrified. It was 
now found that a much lower potential caused the formation of nuclei, 
which gave a very dense form of fogging. Even a glass rod rubbed with 
silk gave sufficient electricity to cause a fog in the test flask, showing that 
only a small quantity of electricity is required, though the potential requires 
to be high. It is not necessary that the salt and iodine should be put into the 
electrifying flask, as the products of the electrifying flask can be drawn into 
the sunning flask with like results. If an almost invisible crystal of salt, 
