=160.= It has been shown that the emanations of thorium and radium behave like radio-active gases, distributed in minute amount in the air or other gas in which they are tested. With the small quantities of active material so far investigated, the emanations have not yet been collected in sufficient amount to determine their density. Although the molecular weight of the emanations cannot yet be obtained by direct chemical methods, an indirect estimate of it can be made by determining the rate of their inter-diffusion into air or other gases. The coefficients of inter-diffusion of various gases have long been known, and the results show that the coefficient of diffusion of one gas into another is, for the simpler gases, approximately inversely proportional to the square root of the product of their molecular weights. If, therefore, the coefficient of diffusion of the emanation into air is found to have a value, lying between that of two known gases _A_ and _B_, it is probable that the molecular weight of the emanation lies between that of _A_ and _B_. Although the volume of the emanation given off from radium is very small, the electrical conductivity produced by the emanation in the gas, with which it is mixed, is often very large, and offers a ready means of measuring the emanation present. Some experiments have been made by Miss Brooks and the writer[253] to determine the rate of the diffusion of the radium emanation into air, by a method similar to that employed by Loschmidt[254] in 1871, in his investigations of the coefficient of inter-diffusion of gases. [Illustration: Fig. 56.] Fig. 56 shows the general arrangement. A long brass cylinder _AB_, of length 73 cms., and diameter 6 cms., was divided into two equal parts by a movable metal slide _S_. The ends of the cylinder were closed with ebonite stoppers. Two insulated brass rods, _a_ and _b_, each half the length of the tube, passed through the ebonite stoppers and were supported centrally in the tube. The cylinder was insulated and connected with one pole of a battery of 300 volts, the other pole of which was earthed. The central rods could be connected with a sensitive quadrant electrometer. The cylinder was covered with a thick layer of felt, and placed inside a metal box filled with cotton wool in order to keep temperature conditions as steady as possible.