Note on the continuous rays observed in the spark spectra of metalloids and some metals

In a paper published in the ‘Proceedings of the Royal Society,’ reasons were given for believing that the back-ground of continuous rays in the spark spectra of the metalloids, for instance, tellurium, arsenic, antimony, and bismuth, was caused by the light emitted by an incandescent oxide, whether in a state of vapour or solid, having its origin in the cooling of the dense vapour of the element in an atmosphere containing oxygen. The spectra of metals which are not oxidisable did not show it, namely, gold, silver, and platinum, neither did those of the easily volatile metals such as mercury, indium, thallium, zinc, and cadmium. It was visible on photographed spectra of metals belonging to the iron group, but at the points of the electrodes only, where a non-volatile oxide is formed. As the original explanation has not been accepted as satisfactory I have recently submitted the question to a special examination.