The absorption is always very strong. However, the rays pass through metals, glass, and paper of slight thickness. One sees that the compounds of the same metal emit rays absorbed to an equal extent. I have obtained good photographic images with uranium, uranous oxide, pitchblende, chalcolite, and thorium oxide. These bodies act at a small distance, whether through air, through glass, or through aluminum.
Thorium sulfate gives weaker images, and potassium fluoxytantalate very weak images.
I have ascertained, moreover, that the uranium, pitchblende, and thorium oxide emit, under the action of R ntgen rays, secondary rays which, from the point of view of discharging electrified bodies, generally have a greater effect than secondary rays from lead. Among the metals studied by M. Sagnac, uranium and thorium come to be placed beside and beyond lead. The uranium used for this study was given by M. Moissan. M. Lacroix was willing to procure for me some mineral samples of known provenance from the collection of the Museum. I thank these gentlemen for their courtesy.