AT one of my demonstrations last week two tubes failed to act. They were both of the kind which depend for their action on a piece of platinum placed within, and from which after bombardment by kathode rays the Rontgen rays take origin. A glance at the tubes showed that owing to the unusual strength of spark the platinums within them were red-hot at the point of impact. Before the demonstration the tubes had been in good working order. I considered they had broken down, but, on returning home, tried them with the spark from my own apparatus, with which they had before answered well. I was somewhat astonished to find them giving off Rontgen rays rather more freely than when first tried. This tends to show that Rontgen rays are not given off by platinum heated above a certain temperature. I think this has already been suggested, but I have not seen it corroborated.