The number of ions in a given volume of air has been the subject of a great many measurements, both at observatories on land and in the course of scientific expeditions at sea. There are ingenious instruments called “ion counters,” in which air is drawn at a measured rate through the apparatus and its electrical effects are noted. The number of positive ions found in a cubic centimeter of the lower atmosphere varies from a few hundred to a thousand or more, while the number of negative ions in the same space is generally one or two hundred smaller. The ionization is about the same over the ocean as over the land. There are several ways in which the air may become ionized. The different rays given off by radioactive substances (Alpha, Beta, and Gamma rays) all have the power of driving off electrons from the molecules of gases; i. e., ionizing them. Air is undoubtedly ionized by radioactive matters in the soil (radium and thorium) and especially by the gaseous “emanations” of these substances in the atmosphere, which are also radioactive. It has, however, been a problem to account for ionization over the ocean; because the amount of radioactive matter in sea water is immeasurably small, while the amount of radioactive emanation in sea air is, according to the latest observations, only about 2.5 per cent of that occurring over the land. The clue to this mystery seems to be found in a special kind of Gamma rays coming from some region far above the surface of the earth. These rays are called the “penetrating radiation,” because they not only are able, like the Gamma rays due to radioactive substances on earth, to pass through the walls of hermetically sealed metal vessels and ionize the air inside, but they also have the power of passing through a great extent of atmosphere without being absorbed. They are estimated to be about ten times as “penetrating” as the Gamma rays coming from known terrestrial substances. The best proof that a radiation of this sort comes from above is that when closed metal vessels are carried up in balloons, there is, above an altitude of about half a mile, a rapid increase in the rate at which ions are produced within them. As to the source of this radiation, one suggestion has been that it comes from strongly radioactive cosmic dust in the upper atmosphere. A hypothesis that seems more plausible at present attributes it to the bombardment of the atmosphere by electrons shot off from the sun.