This, however, is only a beginning. Far more interesting are the results obtained by the study of gases in their relation to the conduction of electricity. As is well known, gases under ordinary conditions are nonconductors. But there are various ways in which a gas may be changed so as to become a conductor; for example, by contact with incandescent metals or with flame, or by treating with ultra-violet light, with Rôntgen rays, or with the rays of a radio-active substance. Now the all-important question is as to just what change has taken place in the gas so treated to make it a conductor of electricity. I cannot go into details here as to the studies that have been addressed to the answer of this question, but I will briefly epitomize what, for our present purpose, are the important results. First and foremost of these is the fact that a gas thus rendered conductive contains particles that can be filtered out of it by passing the gas through wool or through water. These particles are the actual agents of conduction of electricity, since the gas when filtered ceases to be conductive. But there is another way in which the particles may be removed--namely, by action of electricity itself. If the gas be caused to pass between two metal plates, one of them insulated and attached to an electrometer, a charge of positive electricity at high potential sent through the other plate will drive part of the particles against the insulated plate. This proves that the particles in question are positively electrified. The amount of the charge which they carry may be measured by the electrometer. The aggregate amount of the electrical charge carried by these minute particles in the gas being known, it is obvious that could we know the number of particles involved the simplest calculation would determine the charge of each particle. Professor Thompson devised a singularly ingenious method of determining this number. The method was based on the fact discovered by C. T. R. Wilson that charged particles acted as nuclei round which small drops of water condense much as dust particles serve the same purpose. "In dust-free air," says Professor Thompson, "as Aitken showed, it is very difficult to get a fog when damp air is cooled, since there are no nuclei for the drops to condense round. If there are charged particles in dust-free air, however, the fog will be deposited round these by super-saturation far less than that required to produce any appreciable fog when no charged particles are present.