A comparison of the radium emanation spectra obtained by different observers

In 1904 Sir William Ramsay and Prof. Collie gave a list of lines produced by the discharge in a vacuum tube containing radium emanation, but the uncertainty of these numbers made a redetermination desirable. A later determination by Mr. Cameron and Prof. Ramsay was communicated to the Royal Society on June 25, 1908, and was published on August 27, together with corrections, and a final compilation of verified emanation lines added on August 5. After Prof. Rutherford had completed the measurements of the volume of the radium emanation, he and the writer were able to photograph the spectrum that had been observed in the course of this work, and we published in ‘Nature,’ July 9, 1908, the wave-lengths of the stronger lines observed by us in the emanation spectrum, and a more complete list, containing 73 lines, with an accuracy of 0.5 Å. U., was given in the 'Philosophical Magazine’ of August, 1908. Measurements which I have recently made to within 0.1 Å. U. by means of a concave grating confirm the accuracy of our previous determinations. The complete purification of the radium emanation demands a lengthy and painstaking procedure, and is a matter of considerable difficulty, for the volume of pure emanation available in our experiments would occupy at atmospheric pressure not more than one-tenth of a cubic millimetre. The vacuum tube employed must therefore be of small dimensions, and all traces of foreign gases have to be removed from the walls and the electrodes of the tube. In the experiments of Rutherford and Royds, using the method of purification recently developed by Prof. Rutherford, a complete day’s work was taken up before the vacuum tube was filled with the pure emanation.