The isotopic constitution and atomic weights of selenium, bromine, boron, tungsten, antimony, osmium, ruthenium, tellurium, germanium, rhenium and chlorine

In two previous communications* the application of photometry to massspectra was described and results given for krypton, xenon, mercury, zinc, tin, chromium and molybdenum. Observations have been made on eleven more elements and will now be described in the order in which they were successfully performed, together with incidental facts considered worth recording. The methods used were the same as before modified in detail to suit particular difficulties as they arose. A great advance in the technique of the subject was made during the work on boron. This consisted in the application of a new type of photographic plate. Previously the photometry had been carried out exclusively with the standard Paget Half-tone emulsion coated on plate glass. A process by which ordinary plates could be rendered more sensitive was described some years ago,† but as it was impossible to secure either uniformity or keeping qualities such “schumannised” plates were quite useless for photometry. The new plates, which for convenience will be referred to as “Q” plates, are coated with a fast process emulsion of low gelatine content and were prepared in one of the Ilford Company’s laboratories. They are from 5 to 7 times as sensitive to mass rays as the commercial half-tone plate. Their uniformity appears not quite so good, but this is partly due to the fact that they are not yet available coated on plate glass. In any case this disadvantage is largely offset by a steeper and longer linear log range and other desirable qualities. Experiments with Selenium. Of the many methods tried in attacking this element only one has proved practicable. This consists in introducing selenium in the metallic form into the discharge tube at such a point that it is heated by the cathode rays. It then distils to form a coating on the neighbouring walls and this appears to give off sufficient vapour to maintain a suitable discharge pressure.