LIII. The Rate of Transformation of the Radium Emanation. By G. Rümelin, Ph.D. [Communicated by Prof. E, Rutherford, F.R.S] The radium emanation, like all the radioactive products, is transformed according to an exponential law. The "period" of the emanation, or the time required for the emanation to be half transformed, is an important physical constant, an accurate knowledge of which is required in many experiments. Although observations of its period have been made by a number of investigators, the results obtained have not been very concordant. The period of the radium emanation has been determined by Curie [citation redacted], Rutherford and Soddy [citation redacted], Bumstead and Wheeler [citation redacted], and Sackur [citation redacted]; the results obtained by these observers are given below : — Curie 3'99 days Rutherford and Soddy ... 3*77 „ Bumstead and Wheeler ... 3'88 „ Sackur 3'86 Two different methods were used for these determinations. Curie, and Bumstead and Wheeler measured the decay of a quantity of emanation in a closed testing-vessel ; Rutherford and Soddy, as well as Sackur, transferred after different intervals of time into a testing apparatus known volumes of air containing emanation from a large supply which was kept in a gasometer. Since for accurate determinations the measurements have to be extended over several weeks, the first method requires measurements of ionization currents over a wide range of intensity. This is a disadvantage, because the rate of movement of the electrometer-needle, unless very slow, is not accurately proportional to the current, but generally increases more slowly than the latter. The period of the emanation, for this reason, will be found too large. In the second method, this source of error can be eliminated by putting smaller volumes of emanation-air into the testing vessel at the beginning of the experiment than at the end. The drawback of the second method is the liability of escape of emanation, and also of disturbances of the homogeneous distribution during the process of transference of the emanation into the testing-vessel. In the following experiments I endeavoured to get measurements of the period of the radium emanation unaffected by both these sources of error. [header] 551 Let two volumes V1 V2 communicating with each other be filled with air containing Ra-emanation uniformly distributed. Disconnect them; then, if at the time t=0 the smaller volume V1 , when passed into an electroscope, produces the current i1 and at the time t later, the other volume V 2 , when passed into the same electroscope, gives the effect i 2 , it can readily be deduced, that the period T of Ra-emanation calculated from these data is [formula redacted] the theory is based on the assumption that the radium emanation decays exponentially with the time. In order to obtain accurate results it is advisable to make t and v2/v1 large, and arrange the time t between the two observations so as to make the two currents approximately equal. The electroscope is then used under exactly the same conditions, and no comparison of currents of very different magnitude is needed, To eliminate errors due to changes of pressure and temperature of the air, as well as to possible changes of sensitiveness of the gold-leaf system, immediately before the emanation is introduced into the electroscope, the latter is standardized by the [gamma]-rays from a few milligrams of radium bromide, enclosed in an airtight capsule and placed in a definite position near the electroscope. The readings were not taken immediately after the introduction of a quantity of emanation, but three hours later, in order to allow the emanation to reach radioactive equilibrium, when the rate of movement of the gold-leaf is sensibly constant over the time required to make the observations. A quantity of air containing emanation, supplied by boiling some RaBr 2 solution, was collected into a little gasometer over water and thence passed into a partially exhausted glass vessel of the shape shown in fig. 1 . [figure redacted] The vessel was then sealed off at A and brought into a constant-temperature room for about two days, to allow the emanation to distribute itself by diffusion uniformly through the air in the vessel, which was generally at a pressure of about half an atmosphere. By means of a small flame, the 552 [header] two vessels were separated at B, care being taken to protect the other parts of the vessels from radiation from the flame. The air in the smaller vessel V 1 was now transported into the electroscope. The arrangement used is shown in fig. 2. [figure redacted] By means of rubber tubing, connexions were made to allow a current of steam to pass through V1 into a receiving-vessel R containing water, after the narrow ends of the vessel had been opened inside the rubber connexions. The steam was passed through the vessel for some time, in order to completely transfer the emanation into the gasometer R. The electroscope was then exhausted and the collected gas in R allowed to pass in through a tube B containing phosphorus pentoxide and plugs of cotton-wool. Air was then introduced by means of a stopcock at A, which swept the residual emanation in the tube into the electroscope, and brought the air in the latter to atmospheric pressure. Immediately before the introduction of emanation the natural leak was determined and the leak due to the radium standard. The sensibility of the electroscope used was such that 10~ 8 gram radium in equilibrium gave about 8 divisions per minute movement of the gold-leaf on the scale of the reading microscope. The actually measured amounts of radium emanation were in most cases 1*5-4 times larger. Three hours after the introduction of the emanation, the readings were taken, and then the emanation removed. The larger vessel V2 was dealt with in exactly the same way a suitable time later. The volumes of the glass vessels were determined before the emanation was introduced ; the uncertainty due to the sealing off at B (fig. 1) was certainly less than 1/300 of the volume of the smaller vessel. [header] 553 The results obtained by this method are given in the following table : — No. Of experiment V1 in cc V2 in cc I1 in div./min I2 in div./min T in days T in days [table redacted] Each of the numbers marked in the columns i1 and i2 represents the mean of four readings, which never differed from their mean value more than one per cent. The values i1 , i2 , given in the table are corrected for the natural leak, the column i2 also for any slight change of sensitiveness of the electroscope. As it will be seen, the mean of all the experiments, 3*75 days, agrees very closely with the original value obtained by Rutherford and Soddy, 3.77 days. It may be interesting to note that Dr. Bronson in this laboratory recently also determined the period of the radium emanation, using an electrometer steady-deflexion method. Two decay curves of the emanation obtained by him gave the value of T = 3*71 and 3*73 days. It thus seems certain that the period of the radium emanation is not longer than 3*80 days, and very probable that it is in the neighbourhood of 3*75 days. In conclusion, I desire to express my sincerest thanks to Professor Rutherford, at whose suggestion these experiments were undertaken, for his valuable advice during the progress or this work. Macdonald Physics Building. McGill University, Montreal, May 1907.