A short time ago, one of the authors published an account of the effect of temperature on the emanating power of radioactive substances, in the Physikalische Zeitschrift. In the paper it was shown that the emanating power of thorium increased with rise of temperature to about a red heat, but on heating to a white heat the emanating power was destroyed and could not be recovered. An examination of a specimen of radium obtained from De Haen, Hanover, showed that the effect of temperature on its emanating power was very large. When the substance was heated below a red heat, its emanating power increased over 10,000 times, but was to a large extent destroyed by heating to a higher temperature. The emanation, obtained by passing a slow current of air over heated radium, was found to preserve its radioactive powers for weeks, when kept in a closed metal vessel. The radioactivity slowly decayed, but was still quite appreciable after a month's interval. The question now arose, if any physical experiments could be devised to settle the problem as to whether the emanation was in reality a radioactive gas, driven off from the substance, or a vapour of the substance, or a material emission of particles much larger than molecules. Experiments on thorium showed that no appreciable volume of gas could be collected by leaving thorium oxide in a vacuum tube connected with a mercury pump. No new lines were observed in the spectrum of the gas. The amount of the emanation thus given off was thus too small to detect by its volume in this way, but the electrical conductivity produced by the emanation in the gas, with which it is mixed, is often very large and can be used as a measure of the amount of emanation present. The emanation gives out a type of radiation which ionizes the surrounding gas. When a strong electric field is applied, the current through the gas reaches a maximum value, and is then a measure of the total number of ions produced per second, multiplied by the charge on the ion. By determining the rate of diffusion of the emanation into air or other gases, using the electrical method, it is possible to obtain an approximate estimate of its molecular weight. The coefficient of inter-diffusion of most of the simple gases have long been known, and the results show the coefficient of diffusion of one gas into another is inversely proportional to the square root of the product of the molecular weights. If therefore the coefficients of diffusion of the emanation into air is found to have a value lying between that of two gases \(A\) and \(B\), we can conclude that the molecular weight of the emanation lies between the molecular weights of \(A\) and \(B\). The apparatus employed was similar to that used by Loschmidt[1] in his experiments on the coefficient of inter-diffusion of gases in the year 1871.